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What is the minimal focal length of the human eye?

What is the minimal focal length of the human eye?


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What is the minimum length to which the focal length of our eye can go, even when considering the blurred images too.


Short answer
The focal length of the average, healthy, adult human eye at near-point is about 18.5 mm. Young individuals can accommodate their lenses further to a focal length of around 15.4 mm.

Background
The focal length of the human eye is the distance between the lens and the retina when an object is in focus (Fig. 1). Therefore, the… even when considering blurred images toopart in the question doesn't make much sense. So, I will focus my answer on sharp images only (pun intended).

The lenses of the eye are thicker in the center than at the edges, and hence positive and converging lenses. They form an inverted image on the photosensitive layer in the back of the eye - the retina (Fig. 1) The retinal image is shaped by two lenses: 1) the cornea with a fixed focal length and 2) the eye-lens, which is a lens with variable focal length through shape changes (Fig. 1) called accommodation and is mediated by the ciliary muscles (Kolb, 2012).

When the ciliary muscles are relaxed, the focal length of the eye-lens is maximal and the distant objects are in focus (infinity). When the ciliary muscles contract they shorten the focal length of the eye-lens to bring nearer objects into focus. The two limits of this range are called the far-point, (ciliary muscles relaxed), and the near-point (maximal accommodation) (source: University of Colorado, Boulder).

The distance between the eye-lens and the retina is about 20 mm. When an object is far away from the eye (infinity), the image is located essentially at the focal point. Therefore the focal length of the cornea and the eye-lens should be about 20 mm when the muscles of the eye are relaxed. The lens strength is the reciprocal of its focal length in meters. Hence, the strength of the cornea and the eye-lens at the far-point is about 1/0.020 = 50 diopters (source: University of Colorado, Boulder)…

When an object is located at the near-point (the closest point at which an object can be brought into clear focus on the retina), the focal length of the cornea and the eye-lens must be changed so that the image is formed on the retina, which is still 20 mm away. The typical near-point in an adult is 25 cm, corresponding to a focal length of the cornea and the eye-lens of 18.52 mm using the standard ray-tracing rules of lenses. Hence the strength of the cornea and the eye-lens must now be about 1/0.01852 = 54 diopters. In other words, the muscles of the eye can provide an accommodation range of 4 diopters (source: University of Colorado, Boulder).

Kids can, however focus to points as close as 6.5 cm away, i.e., 15 diopters worth of optical power, i.e., a minimal focal length of 15.39 mm.


Fig. 1. Human eye. Upper panel: non-accomodated relaxed eye. Lower panel: accomodated eye. source: Khan Academy

Reference
- Kolb, Gross Anatomy of the Eye. In: Webvision. The Organization of the Retina and Visual System, Moran Eye Center (2012)


Closest focusing distance of a human eye?

Was wondering, so I just measured mine. Focuses up to 14 cm close. Yours?

Twelve centimeters for me, but then I'm extremely nearsighted--my maximum focusing distance is about 15 centimeters. My vision sans augmentation is 20/400+

"Tell me the smallest letter on the chart that you can see clearly."
"There are letters on the chart?"

RDKirk
'TANSTAAFL: The only unbreakable rule in photography.'

. just poked myself in the eye with a ruler.

Closest Focusing Distance for me
Friday 6:00 PM: 7"
Friday 10:00 PM: 5"
Saturday 1:00 AM 3"

Well you are all younger than me (40) or my eyesights really bad. At my last eye test (3 months ago) i could read the smallest line easily but today i can only focus at around 20cm, although it all depends how much your lens has hardend. 24 years Professional Letterpress/Litho printer.

saaketham1 wrote:

Closest Focusing Distance for me
Friday 6:00 PM: 7"
Friday 10:00 PM: 5"
Saturday 1:00 AM 3"

Friday 6:00 PM: 7"
Friday 10:00 PM: 5' (Feet)
Saturday 1:00 AM (wish on o' them 3 shulld i focusss on??)

and other things. Young adults with 20/20 vision can often focus at 4 to 6 inches. At 6 inches, they can see detail at 600 pixels per inch or 300 line pairs per inch. As you age, your lenses become less resilient and the focus distance increases. At about 50 years, the average minimum focus distance is about a foot corresponding to 150 line pairs per inch or 300 pixels per inch. These numbers reflect good light and high contrast scenes. Generally, under more typical conditions, people don't see this well.
--
Leon
http://homepage.mac.com/leonwittwer/landscapes.htm

Dash29 wrote:

Well you are all younger than me (40) or my eyesights really bad.

I'm older than you by more than a decade.

I'm both nearsighted (myopia) AND farsighted (presbyopia). My uncorrected range of vision is 12-15 centimeters.

I normally wear bifocals that give me normal reading distance vision from the lower half of the lens and normal distance vision from the upper half of the lens. When I have to see close up, I peer over the top of the glasses to get my unaugmented super-close vision.

My interpupillary distance is also wider than normal--80mm, which according to my optometrist should be giving me a bit more accurate stereo vision than normal. My daughter calls this a "useless super power."

RDKirk
'TANSTAAFL: The only unbreakable rule in photography.'


Why did some ichthyosaurs have such large eyes?

Many species of extinct marine ichthyosaurs had much larger eyes for their body size than would be expected of extant marine mammals and reptiles. Sensitivity to low light at great depth for the deep-diving genus Ophthalmosaurus has recently been suggested as the reason for the large eyes of these animals. Here, we discuss the implications for vision at such depths and consider other optical factors determining eye size. We suggest that the large eyes of ichthyosaurs are more likely to be the result of simultaneous selection for both sensitivity to low light and visual acuity. The importance of the evolutionary history of extant marine mammals and extinct ichthyosaurs is discussed, as are ecological factors driving both acuity and sensitivity.

Ichthyosaurs were large marine reptiles that lived between 90 and 250 million years ago. Fossil evidence suggests that several species had very large eyes in comparison with those of the extant dolphins, with which ichthyosaurs are often compared. For example, some 9 m long ichthyosaurs had eyes 25 cm in diameter, more than five times that of similar-sized extant marine mammals. Recently, on the basis of estimating the f-number (see below) of the eye, it has been suggested that the particularly large eyes of the genus Ophthalmosaurus allowed it to see in the low light conditions experienced in the sea at depths of at least 500 m (Motani et al., 1999). This depth estimate is similar to those of the dive depth for this genus, based on scaling relationships between size and swimming speed and on size and dive duration in extant diving animals (Motani et al., 1999). Here, we re-evaluate the methods used to obtain these depth estimates and consider the implications of this revision. We suggest that previous estimates may be even more interesting than they first appear.

First, experiments with seals at low light levels suggest that harp seals (Phoca groenlandica) are sensitive to different visual images at light levels equivalent to those experienced at a depth of approximately 615 m (Lavigne and Ronald, 1972). In a similar experiment, Wartzok (1979) reported a value of 670 m for spotted seals (P. largha). Since seals do not have unusually large eyes compared with those of other mammals, this suggests that ichthyosaurs may well have been able to see at depths substantially greater than 500 m without recourse to enlarged eyes.

The argument that large eyes suggest deep diving is based on the estimation of the f-number of the eye, which is the ratio of the focal length (lf) of the optical system to the diameter of the aperture (da) through which light enters (Denny, 1993). Thus:

The sensitivity of the eye (S) changes with f-number to the power –2:

where L is the radiance (which is approximately equal to the brightness) of the source. Hence, low f-numbers lead to high sensitivity. We have been able to estimate an f-number for an elephant seal (Mirounga spp.) eye and, depending upon assumptions about lens size, we estimate that the minimum f-number for this species is between 1.18 and 1.48 (see Appendix). Motani et al. (1999) estimated the f-number of Ophthalmosaurus to be 0.76. Hence, all other things being equal, Ophthalmosaurus would have had a sensitivity 2.5–4 times that of an elephant seal. The largest of these values suggests that Ophthalmosaurus could probably see in light levels approximately 25 % of the minimum requirements of the elephant seal. Surprisingly, this greater sensitivity buys only 42 m of extra depth, since light intensity in the oceans decreases by approximately 90 % for every 70 m dropped (Wartzok and Ketten, 1999). Considering that elephant seals are known to forage at depths of over 1000 m (Schreer and Kovacs, 1996), the comparatively small potential expansion of depth range that large eyes would bring suggests that visual sensitivity alone is insufficient to explain why these ichthyosaurs had huge eyes.

However, we must also be mindful that the method used to estimate the f-number of Ophthalmosaurus by Motani et al. (1999) is necessarily indirect and speculative because they were forced to make assumptions based only on preserved skeletal material and not soft tissues. Extant fish, squid and seals, in which we can examine optical systems directly, all have a ratio of focal length to lens radius of approximately 2.5 (Mattheissen’s ratio), which equates to an f-number of 1.25 (Land, 1981). The convergence on Mattheissen’s ratio through the different evolutionary pathways followed by these groups indicates that 1.25 is likely to be the minimum achievable f-number (Land, 1981). This suggests that the estimate of Motani et al. (1999) of 0.76 for Ophthalmosaurus may be a considerable underestimate. However, even if Ophthalmosaurus had an f-number of 1.25, it would still have been able to detect light usefully at considerable depths. The human eye, with a fully open pupil, has an f-number of 2.0 (M. F. Land, personal communication) which, all other things being equal, makes it 2.6 times less sensitive than a fish eye. The absolute threshold of the human eye is approximately 10 log units lower than the intensity of sunlight at the ocean surface, meaning that humans are able to see to a depth of approximately 700 m. In comparison, a fish (or Ophthalmosaurus) with an f-number of 1.25 and an eye equivalent in size and retinal structure to our own, would be able to see to a depth of perhaps 750 m. Hence, this line of reasoning also suggests that sensitivity to low light levels alone seems unlikely to provide a full explanation for the large eyes of ichthyosaurs.

Sensitivity to low light levels is only one measure of visual ability another is the ability to resolve fine detail in an image (visual acuity). The resolving power (R) of an eye increases with the focal length of its lens (Bradbury and Vehrencamp, 1998) as:

where dr is the centre-to-centre spacing between the receptors of the retina. This introduces a trade-off, since increasing the focal length of the eye on its own increases the f-number and so decreases sensitivity. One way to achieve both good sensitivity and acuity is to allow the focal length to increase, but simultaneously to increase the aperture size to avoid increasing the f-number. Hence, it may be that the large eyes of ichthyosaurs were a result of simultaneous selection for both high sensitivity and acuity. However, it is interesting to note that the visual acuities of extant cetaceans and pinnipeds are generally good and comparable with those of terrestrial hunters such as the domestic cat Felis catus (Muir and Mitchell, 1973).

Visual performance also depends on retinal pooling – the summation of signals from individual sensory cells to produce a retina with fewer individual receptor units but greater sensitivity per receptor. With the longer focal length of its larger eye, an ichthyosaur could pool signals over a much larger region of retina, without loss of acuity, than humans. Alternatively, it could trade off some acuity in return for even greater sensitivity. Land (1981) has suggested that eye size is proportional to the product of resolution and the square root of sensitivity. Hence, increasing resolution by a given factor requires a greater increase in eye size than the same relative increase in sensitivity. This, combined with the impressive visual performance of extant aquatic mammals without huge eyes, suggests that the large eye size of ichthyosaurs was driven by a need for greater visual acuity allied to sensitivity to low light levels. This seems especially likely because the logarithmic decrease in light intensity with depth means that, at depths below 500 m, considerable improvement in sensitivity is required to produce an ecologically relevant increase in the range of visible depths.

However, in terms of visual acuity, the type of receptor cell predominating in the retina strongly influences the value of dr, as these cells determine the level of receptor pooling. In general, rods tend to pool signals across several neighbouring receptors, thus effectively increasing the value of dr, while cones generally do not pool. Thus, a predominance of cones in the retina suggests that the value of dr is relatively small and, hence, that resolution is relatively high (Walls, 1963). This difference can be explained by the function of the two receptor types. Rods are generally found in animals adapted to low light levels, while cones predominate in diurnal species. The phylogenetic history of ichthyosaurs and extant marine mammals indicates that the former were derived from primarily diurnal reptilian ancestors, while mammals are characterised by nocturnal predecessors (Walls, 1963 Muntz, 1978). This suggests that ichthyosaurs had visual systems already geared towards visual acuity more than sensitivity. Pooling of receptor signals in ichthyosaurs would allow increased sensitivity, but at the cost of reduced acuity. Thus, relatively large eyes would appear to be an adaptation for both acuity and sensitivity in these animals.

The above arguments lead us to the conclusion that the ecological demand giving rise to the large eyes of the ichthyosaurs was not simply a need to see in the low light environment of the ocean depths. Rather, large eyes probably developed in response to the constraint of sensitivity, in conjunction with a need for high visual acuity. However, the mechanism driving this need for high visual acuity is not obvious, especially given the good acuity of modern marine mammals. One possible hypothesis is that the main predators and prey of the ichthyosaurs were superficially similar in appearance at a distance, and fine resolution was required to tell one from another at sufficient range to allow flight from predators. However, this is an odd situation apparently not encountered by extant animals, especially considering the body-size scaling relationships involved in predator/prey systems. A more plausible explanation for this need for both sensitivity and acuity is that these animals were fast, active hunters of small prey at some depth. A similar argument involving the amount of receptor-cell pooling might explain the occurrence of relatively large eyes in many extant cephalopods, such as the giant squid Architeuthis, that are fast, deep-swimming hunters. A further possible consequence of selection for high visual acuity is the use of visual signalling or individual recognition between ichthyosaurs, perhaps related to mating or coordinated foraging. It is noticeable that marine animals that do have primarily visual communication (e.g. many cephalopod molluscs, mantis shrimps) also have large eyes relative to their body size.

In summary, we suggest that the large eyes of Ophthalmosaurus are the result of simultaneous pressure for sensitivity, allowing prey detection at considerable depths, combined with pressure for high acuity, allowing these animals to hunt small, fast-moving prey.


Focal length eye

In subsequent sections, a 58-D reduced eye model is used, that is, an eye with a single refracting surface that separates air from aqueous humor with an index of refraction of 1.333. The radius of curvature this eye equals 333/58 = 5.74 mm, its first focal length = 1000/58 = 17.2 mm, and its length or second focal length = 1333/58 = 23.0 m . The more commonly accepted value, however, is 22mm to 24mm.. When you look through a viewfinder, a lens at around 50mm focal length will show objects at the same size as when you look at something with your eyes. You could test this by looking through the viewfinder with one eye, and looking next to it with the other eye. When you close one of your eyes, you will notice that your sight does not change, regarding the size of objects. This applies to APS-C cameras, as well for full frame cameras

For the popular 35 mm film format, typical focal lengths of fisheye lenses are between 8 mm and 10 mm for circular images, and 15-16 mm for full-frame images. For digital cameras using smaller electronic imagers such as 1⁄4 and 1⁄3 format CCD or CMOS sensors, the focal length of miniature fisheye lenses can be as short as 1 to 2 mm The focal length of the average, healthy, adult human eye at near-point is about 18.5 mm. Young individuals can accommodate their lenses further to a focal length of around 15.4 mm We created Lumion so the focal length corresponds with 35mm full frame. This makes it easier to look up the desired focal lengths. A focal length which comes close to the human eye is about 50mm. This is just a common value and I'm not sure this is an exact scientific value. Well actually I'm pretty sure it's not. It's probably what people. Again, too specialized.

24mm Well that focal length is neither fish nor fowl, even though Canon's and Nikon's F2.8 standard zooms start at 24mm. Panasonic has the exact equivalent with its 12-35mm F2.8, whereas Olympus lenses seem to generally like a base focal length of 12mm a.k.a. 24mm The focal length of a lens used on a digital camera, is the distance between the focal point of a lens and the sensor when the subject is in focus. It doesn't refer to the size of the lens, because it's not the actual length of the lens. The focal point is inside the lens, at the point where light rays converge

Based on this, I'd like to propose that before you latch onto focal lengths such as 22, 24 or 50mm as the closest focal length to the human eye, I strongly suggest that you think first about what you actually want to capture in your image. Seeing and Perceiving. How we see and how we perceive the world, are two very different things. A 24mm focal length might be great for photography when you want to show approximately the amount of the scene that we can see with our peripheral. Although the human eye has a focal length of approximately 22 mm, this is misleading because (i) the back of our eyes are curved, (ii) the periphery of our visual field contains progressively less detail than the center, and (iii) the scene we perceive is the combined result of both eyes. Each eye individually has anywhere from a 120-200° angle of view, depending on how strictly one defines. length of 50 mm provides a very close field of view of the vision of our eye, hence having also a magnification of 1. In fact, and for the purists, this focal length is 43 mm. We'll just use the very good approximation of 50 mm for the rest of our words. From there, we can say that the magnification of a lens is equal to the focal length divided by 50. with α = angular size of image β.

However, oculars with small focal length tend to have a smaller eye relief, e.g. only 2 or 3 mm, although there are design methods with which more can be achieved - possibly at the expense of other parameters. A particularly large eye relief is required for riflescopes, because the recoil would otherwise push the ocular into the eye. Ocular Lens Designs. There is a wide range of optical. For a similar perspective to the human eye you need something between 40mm and 55mm focal lengths in 35mm frame terms A fast (i.e., low focal ratio) telescope can have a fairly long focal length, if the aperture is large enough, a 24 f/3.3 for example. In that case, a low-power (i.e., long focal length) eyepiece will produce an overly large exit pupil. #5 macdonjh Fly Me to the Moo . You can zoom in on your phone, but that's not changing your focal length. That's just cropping your photo before you actually take it, photographer Derek Boyd points out

Larger focal length lenses have less optical power. In SI, optical power is measured in reciprocal meter (m⁻¹). This unit is usually called the dioptre or diopter. For example, a 2-diopter lens can focus parallel rays of light at ½ meter. We can view the above formula in action if we dive without a mask or goggles: we cannot see clearly because the refractive index of water at 20° C is 1. The focal length of your eyepiece is often printed on the eye piece itself. If your telescope has a focal length of 800mm and you are using a 20mm eyepiece you divide the focal length of the scope by the focal length of the eyepiece: 800mm/20mm = 40. As a result you will get 40X as a your magnification

The maximum focal length of eye lens is 2.5 cm. The distance between lens and retina is 2.5cm.Minimum focal length occur when you focus on images at your nearpoint The minimum focal length of eye lens is 2.27 cm Perhaps the starting point for equivalence with the human eye is the focal length there are various answers to this question and ClarkVision provides a good summary that can be distilled down to. Focal Length. This schematic shows an example of a convex lens on the top and a concave lens on the bottom. The focal point (F) is the point at which parallel light rays cross Your choice of Focal Length will have a dramatic affect on how your story is told. This video covers some general uses for various focal lengths in order to..

Focal Length Eye Photography On Landscap

  1. imum limit. T ry to read a printed page by holding it very close to your eyes. Y ou may see the image being blurr ed or feel.
  2. What is Focal Length? Ultra Wide Angle Lens [10mm to 24mm]. As the name suggests, these lenses have a very wide angle of view! Often used by. Wide Angle Lens [24mm to 35mm]. Wide angle lenses are not quite as wide as the ultra wides, but are still quite wide. Standard Lens [35mm to 70mm]. Standard.
  3. This principle is used in the eye's lens. The focal length is somewhat reduced for focusing on nearby objects. When an optical system contains multiple optical elements (e.g. lenses), the focal length may be tuned by adjusting the relative distances between the optical elements. This principle is used e.g. in photographic zoom objectives. Wavelength Dependence of the Focal Length Using Curved.
  4. The focal length (f) is the distance between the lens and the focal point. Because the focal length measures a distance, it uses units of length, such as centimeters (cm), meters (m), or inches..
  5. It is accepted that the human eye has a magnification of 1. It is also generally accepted that a lens with a focal length of 50 mm provides a very close field of view of the vision of our eye, hence having also a magnification of 1. In fact, and for the purists, this focal length is 43 mm. We'll just use the very good approximation of 50 m
  6. The distance from the magnifying lens to the piece of paper is the focal length. For the eye, light from distant objects is focused onto the retina at the back of the eye. The eye is about the size of a table tennis ball, so the focal length needs to be about 2.5 cm. The cornea does most of the focusing . About 70% of the bending of light takes place as it enters the cornea and the aqueous.
  7. Without getting too technical, focal length can be defined as the distance between the optical center of the lens and the image plane (the sensor or film) when the lens is focused at infinity. Focal length is typically measured in millimeters, and is the primary defining trait of a lens

When the eye is relaxed and the interior lens is the least rounded, the lens has its maximum focal length for distant viewing. As the muscle tension around the ring of muscle is increased and the supporting fibers are thereby loosened, the interior lens rounds out to its minimum focal length. Things like this are because of the curvature of the lens of your eye. If it's too curved the focal point of your eye will land somewhere in front of your retina causing myopia, or short-sightedness, and in the case of your lens being not curved enough, the focal point lands behind the retina and you have hyperopia or long sightedness PS EYE: Focal Length / Brennweite ? Diskutiere PS EYE: Focal Length / Brennweite ? im Hardware und Zubehör Forum im Bereich Playstation 3 Hallo Leute! Vielleicht ne doofe Frage, aber kann mir jemand sagen, welche Brennweite/ Focal Length die PS Eye Kamera hat? Vielen Dank schon mal! Neues Thema erstellen Antworten 04.03.2011 #1 H. hoodoo101. Dabei seit 04.03.2011 Beiträge 1. Approximating the eye as a single thin lens 2.60 cm from the retina, find the eye's near-point distance if the smallest focal length the eye can produce is 2.20 cm. Solution: Chapter 27 Optical Instruments Q.5C

Thus, focal length is the distance behind the lens at which collimated light striking the lens will converge. For compound lenses (lenses with more than one lens element with real thickness - pretty much every modern photographic lens), the distance a theoretical thin lens having the same refractive properties would need to be in front of the focal plane for collimated rays striking that lens. Focal length is the distance (measured in millimeters) between the point of convergence of your lens and the sensor or film recording the image. The focal length of your film or digital camera lens dictates how much of the scene your camera will be able to capture Do this for every shot to make sure their eye is roughly in the same place for each photo. As your focal length changes you'll need to adapt. For each new focal length, starting at 28mm, you'll have to move further back and realign your subject in the frame as closely as possible to your first phot The standard focal length lens (50 mm for the 24 x 36 m format) is the standard as it very well approximates the human eye's FOV. Generalized, a focal length of about 115 % of the image's. Focal lengths with larger numbers make subjects appear larger compared to how the human eyes perceive them. Moreover, the longer the focal length of a lens, the more elements stack within the frame, causing a photo to have a compressive perspective. These focal length lenses can create a shallow depth of field, allowing you to focus on small objects at particular distances or make distant subjects closer

What Is The Normal Focal Length Of Human Eye? - Make money

  1. The principal focal point - nearest point For an object at 250mm from the eye the principal focal length would be: 1 f = 1 do + 1 di 1 f = 1 d o + 1 d i 1 f = 1 250 + 1 20 1 f = 1 250 + 1 2
  2. If the object distance is changed (i.e., the eye is trying to focus objects that are at different distances), then the focal length of the eye is adjusted to create a sharp image. This is done by changing the shape of the lens a muscle known as the ciliary muscle does this job. Nearsightedness . A person who is nearsighted can only create sharp images of close objects. Objects that are.
  3. Focal length is the distance between the optical center of the lens, and the camera sensor or film plane when focused at infinity. The optical center is where light rays converge inside the body of your lens. The focal length defines the magnification and field of view for a given lens. This value is most commonly measured in millimeters. Prime lenses have set focal lengths whereas zoom lenses.

For a normal eye, when the eye muscles are relaxed, the focal length f of the lens L is slightly less than its diameter, which is D = 2.4 cm. An idealized eye will focus an object at infinity on the retina which is located at distance, D, behind the lens of the eye (see Figures 5 and 6a below). When the eye views a closer object, the eye muscles produce a shortening of f (so-called. What is Lens Focal Length. Focal length, usually represented in millimeters (mm), is the basic description of a photographic lens. It is not a measurement of the actual length of a lens, but a calculation of an optical distance from the point where light rays converge to form a sharp image of an object to the digital sensor or 35mm film at the focal plane in the camera. The focal length of a lens is determined when the lens is focused at infinity Focal length is measured in millimeters (mm) and it represents the distance from the optical center of a lens to the digital camera sensor when the subject of the photo is in focus. This is the standard textbook definition, but it's still not entirely obvious WHY you need to know about it before your purchase a new lens The distance from the lens to this principal focus point is called the focal length of the lens and will be designated by the symbol f. A converging lens may be used to project an image of a lighted object What is the focal length of the human eye related to 35mm? I heard that to get the picture with closest perspective to the one of the human eye, the focal length of the lenses should be set to the focal length of the human eye, is that true? ibiza123's gear list: ibiza123's gear list. Fujifilm FinePix HS10 Sony Cyber-shot DSC-HX90V Panasonic Lumix DMC-GX7 Panasonic Lumix G Vario 14-140mm F3.5.

What's the focal length of a human eye? - Quor

  1. 1. Eye piece have moderate focal length. If it is decreased after a certain limit, the focus of eye piece would be very close to the optical centre of the eye piece and therefore the eye piece would suffer spherical aberration. The image formed by the eye piece would no longer be clear. Share
  2. A normal human eye can clearly see all the objects at the different distance. Reason The human eye has the capacity to suitably adjust the focal length of its lens to a certain extent
  3. Focal length is the system used in photography to describe how wide or tight a lens is. Listed as a number and measured in millimetres — e.g., 35mm, 85mm — it tells you how much of a scene a lens can capture, and how big subjects will appear. The number gives an indication of the angle of view that a lens can see
  4. Focal length is not stated directly in a prescription for eyeglasses. Instead, the refractive power is used to describe the extent to which a lens refracts light. The formula used to find the refractive power of the lens (in diopters) is the inverse of the focal length (f: given in meters). This relationship shows that the greater the power of a lens, the shorter the focal length. For example.
  5. Page 1 of 2 - Eyepieces - Focal Length vs FOV vs Eye relief - posted in Beginners Forum (No astrophotography): Eyepiece descriptions seem somewhat confusing: 1. Ive read that some expensive eyepieces give a wider field of view at the same magnification than cheaper eyepieces. Is this true? 2. Also read about eye relief being measured in mm - How, if at all, does this relate to focal length
  6. e how much of the subject your camera sees. You may already be familiar with the basics, and understand the difference between, say, wide-angle and telephoto lenses, but let's dive into the the topic a little deeper to see what's really going on. There are four fundamental things to know and.
  7. g image has a focal-length equivalent to 22-24mm

The focal length of the lenses of an astronomical telescope are 50 c m and 5 c m. The length of the telescope when the image is formed at the least distance of distinct vision is 9. The radius of curvature of each surface of a convex lens of refractive index 1.5 is 40 c m 25mm (50mm): This focal length is said to be identical to the human eye. The image is what you see with your eyes, in terms of distance. When shooting a close up from a face, this is the minimum focal length you want to use Q: If the focal length of a magnifier is 5 cm calculate (a) the power of the lens (b) the magnifying power of the lens for relaxed and strained eye. Sol: (a) As power of a lens is reciprocal of focal length . #92large P = \frac<1><5\times 10^<-2>> = \frac<1><0.05>$ P =20 D (b) for relaxed eye, MP is minimum and will b The answer to this question is: When the muscles are relaxed, the lens becomes thin. Thus, its focal length increases. Access a diverse Question Bank and ask You Own Doubt Now

The human eye - University of Tennesse

Focal Length and Depth of Focus. In addition to f-stop affecting depth of focus, focal length plays a significant role in depth of focus. The greater the focal length, the less the depth of focus. Stated another way, the more you zoom in, the less objects in the background will be in focus. As noted below, the image of the flower on the right. Many translated example sentences containing focal length of eye - Spanish-English dictionary and search engine for Spanish translations A person with normal near point $25\, cm$ using a compound microscope with objective of focal length $8.0\, mm$ and an eye piece of focal length $2.5\, cm$ can bring an object placed at $9.0\, mm$ from the objective in sharp focus. The separation between two lenses and magnification respectively ar

This can't be done with the human eye: the image distance, the distance between the lens and the retina, is fixed. If the object distance is changed (i.e., the eye is trying to focus objects that are at different distances), then the focal length of the eye is adjusted to create a sharp image. This is done by changing the shape of the lens a. Eyepiece Focal Length Calculators The calculators below perform a rough determination of the Effective Focal Length (EFL) of simple 2- and 3-element eyepieces. You need only specify the focal length of the individual elements and the spacing between the elements. The formulas employed by these calculators are approximations Medium Focal Lengths. Medium focal lengths fall anywhere within the 35mm to 70mm range. This range is the most similar to what we see with our own eyes. In general, human eyesight is equivalent to about 50-70mm on a full frame camera. This focal length is great for walking around. You can frame photos quickly as the image will largely resemble. Calculation of focal length and magnification of a magnifying glass. Conversion factor is the near point distance of the eye, which is estimated as 25 cm. This is not the distance of the glass from the object A telescope has magnification 5 and length of tube 60 cm then the focal length of eye piece is- asked Jan 11, 2020 in Physics by Nishu03 (64.2k points) jee main 2020 0 votes. 1 answer. The magnifying power of a small telescope is 20 and the separation between its objective and eye piece is 42 cm in normal setting. asked May 7, 2019 in Physics by Sweety01 (70.0k points) optics jee jee.

focal length Bedeutung, Definition focal length: the distance between a point where waves of light meet and the centre of a lens focal length definition: 1. the distance between a point where waves of light meet and the centre of a lens 2. the distance. Learn more The probe is equipped with a 60 mm focal length lens, with optional 80 mm and 120 mm focal length lenses available. tsi.com Die Sonde is t mit einer Linse mit e in er Brennweite von 6 0 mm ausgestattet, o pt ion al sind Linsen mit 80 und 1 20 m m Brennweite e rhäl tl ich

A telescope with objective of focal length 60 cm and eye piece of focal length 5 cm is focused on a far distance object such that the parallel rays emerge from eye piece. If object subtends an angle 1 o on objective, then angular width of the image is : (A) 62 ° (B) 48 ° (C) 24 ° (D) 12 ° This PR uses the eye camera intrinsics (specifically focal_length) to get a more accurate 3D eyeball position estimate. Depends on pupil-detectors v1.1.1, upgrade with pip install -U pupil-detectors Summary Added known intrinsics for the eye cameras to camera_models.py On recording stop, current eye camera intrinsics are saved to the recording (like with world.intrinsics) The 3D detector is. we've been doing a bunch of these videos with these convex lenses where we drew parallel rays and rays that go through the focal point to figure out what the image of an object might be but what I want to do in this video is actually come up with an algebraic relationship between between the distance of the object from the convex lens the distance of the image from from the convex lens usually.

Muchos ejemplos de oraciones traducidas contienen focal length of eye - Diccionario español-inglés y buscador de traducciones en español The focal length of a lens f is the distance from a lens to the focal point F . Light rays (of a single frequency) traveling parallel to the optical axis of a convex or a concavo-convex lens will meet at the focal point The eye has a nominal focal length of approximately 17mm,[1]but it varies with accommodation. The nature of human binocular vision, which uses two lenses instead of a single one, and post-processing by the cortex is very different to the process of making and rendering a photograph, video or film

The Human Eye as an Optical System Ento Ke

  • As the traditional wideangle lens used to have a focal length of around 28mm, most kit lenses start from 18mm to meet this length (ie, 18 x 1.5 = 27mm). This is equally the case for the Four Thirds format, whose 2x multiplication factor means that Olympus's kit lenses start at 14mm. As only the central part of the image is used by the sensor, it allows digital lenses to be both lighter and smaller, as less glass is needed in their construction
  • The next time you're out shooting for fun, limit yourself to one focal length. Ideally, the focal length you choose will be one extreme or the other (telephoto or wide) so that you're forced to see the world through different eyes than normal. If you're using a zoom, keep it set to one focal length the entire time
  • So, what is the average focal length of the average human eye? The size of a human adult eye is approximately 24.2 mm (transverse) × 23.7 mm (sagittal) × 22.0-24.8 mm (axial) with no significant difference between sexes and age groups
  • Focal length= 1/2center of curvature The Attempt at a Solution a) C=1/2D =1/2(2.5cm) =1.25cm F=1/2C =1/2 (1.25) =0.625 It doesn't make sense though, that focal length seems too small/odd. It says in the question that it changes to between 2.1 and 2.3, so my answer seems wayyyyy off. http://img23.imageshack.us/img23/9751/eyecopyw.jpg [Broken

The Camera Versus the Human Eye - PetaPixe

closer to the eye than 73 cm. Determine the focal length of contact lenses that will enable this person to read a magazine at a distance of 25 cm. Solution: Remember a converging lens is required to correct far sightedness. That is, the focal length of the lens is positive. Far sighted-objects close up are blurred A diopter is defined as a unit of lens power equal to 1/focal length of the lens in meters, or 100/focal length of the lens in centimeters, or 40/focal length of the lens in inches. In eye care, by convention, we work in quarter diopter units of power. Slide 3 Corrective lens are either positive or negative in power. Negative or minus lenses cause light to diverge. Positive or plus lenses cause light to converge Longer focal lengths really limit your field of view in my opinion. Imagine a completely black room, your only contact with the world is trough a small window and a large one. Of course, the large one is the 28mm. I don't know about you, but I want as much of the world that I can get. When you use a 28mm, it's as much about the subject that you are shooting, as it is their background. I have seen optical models of the eye, in the OSA Handbook, for example. From memory, the eye is about 25mm diameter making its lens power 40D. add your prescription algebraically to to get 28.75D. Take the reciprocal to obtain a focal length of about 35mm. That is an indication of how long your eyeball is. It is possible to calculate more.

Field of view - What lens focal length most closely

As this look is considered to resemble the focal length of the human eye, Ozu used it to create a naturalistic approach to his story. Another more recent example of single-lens use in a film is Call Me By Your Name, directed by Luca Guadagnino. Luca Guadagnino with his director of photography Sayombhu Mukdeeprom took on the challenge of filming the entire film with a 35mm lens. Their aim was. The standard lens has a fixed focal length (50mm, 85mm, 100mm), and reproduces fairly accurately what the human eye sees - in terms of perspective and angle of view. For a 35mm film camera or a full-frame DSLR, the 50mm lens is considered standard The crystalline lens of the human eye is a double convex lens made of material haveing an index of refraction of 1.44. Its focal length in air is about 8 mm which also varies. We shall assume that the radii of curvature of its two . Physics. An object is placed 30mm in front of a lens. An image of the object is located 90mm behind the lens. a) Is the lens converging or diverging? explain. b) What is the focal length of the lens? c) Draw a diagram with lens at x=0 The length focal length is calculated using the following formula: 1 6 + 1 7 = 1 U and V are measured from the principal planes. Ru and Rv are measured from the lens vertexes. If the object O is close to the front focal point, the beam coming out of the lens is almost collimated and Rv is very large. Thus, d << Rv, and one can approximate V

Rv. Rv is measured for two positions U1 and U2.

Fisheye lens - Wikipedi

Focal, length, eye, focus icon. Open in icon editor. This is a premium icon which is suitable for commercial work: Use it commercially. No attribution required. Comes in multiple formats suitable for screen and print. Ready to use in multiple sizes. Modify colors and shapes using the icon editor. Add icon to cart $2.00 Focal length. Focal length, or focal length range in the case of zooms, will usually be the foremost consideration when choosing a lens for a specific photograph or type of photography. The focal length of a lens determines two characteristics that are very important to photographers: magnification and angle of view When it comes to photography, normal most often refers to the standard focal length lens on a camera. A normal lens sees about the same angle of view as the human eye. Let's delve into what normal means and why it's important. A normal lens is one whose focal length is the diagonal of the sensor of the camera. The sensor size is commonly known as the format. A full frame DSLR sensor. Focal length choice is a huge part of the composition process of an image. You can use a wide lens to lead into a background or create distance, or choose a longer focal length to compress your subject against the background. A focal length of any choice can be a good one depending on the way you envision the scene

Vision - What is the minimal focal length of the human eye

Those buying a point-and-shoot camera for landscape photography also should keep an eye on focal length equivalent to make sure their camera can go wide enough. Ideal Focal Lengths for Landscapes. Focal length(s) is perhaps the most important factor in choosing a landscape lens. As we mentioned above, the heart of the landscape focal length range is 14mm to 35mm. To illustrate what each focal. Describes how focal length is defined for a converging or diverging lens. About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new. Focal length is something that we talk about constantly as we discuss different lenses and styles of photography in our weekly free podcasts.. It can be a bit confusing as a beginner to understand focal length because there are a few twists and complexities, but I'll do my best to explain it in 5 minutes or less

Changes in focal length of a camera lens does not directly influence perspective. An alternative, but equivalent statement would be: 4. The appearance to the eye of objects in respect to their relative distance and positions. From Wikipedia. Perspective, in the context of vision and visual perception, is the way in which objects appear to the eye based on their spatial attributes or their. Standard focal lengths range from 35mm to 50mm depending on the type of camera sensor. The field of view provided by standard focal lengths approximates the field of view of the human eye. Images taken with a standard focal length show a natural perspective without distortions >>> so it makes sense that he would use 50mm lenses for entire films since the 50mm (as well as the 35mm) is often considered to resemble the focal length of the human eye. This well known 50mm lens comment is in reference to the 35mm Full Frame still photo format, which would translate to a 35mm lens for the Super35 film format T he primary measurement of a lens is its focal length. The focal length of a lens, expressed in millimeters, is the distance from the lens's optical center (or nodal point) to the image plane in the camera (often illustrated by a Φ on the top plate of a camera body) when the lens is focused at infinity. The image plane in the camera is where you. The human eye has a focal length of somewhere between 40mm and 58mm, with 50mm being the usual compromise. This is referred to as the normal focal length . It's hard to measure because a camera lens is not a perfect analog of our eyes Other articles where Focal length is discussed: photoreception: Diversity of eyes: lens surface, which shortens its focal length (the distance from the retina to the centre of the lens). One of the most interesting examples of amphibious optics occurs in the four-eyed fish of the genus Anableps, which cruises the surface meniscus with the upper part of the eye looking int


Electromagnetic Spectrum and Color

Visible light is just one form of electromagnetic radiation (EMR), a type of energy that is all around us. Other forms of EMR include microwaves, X-rays, and radio waves, among others. The different types of EMR fall on the electromagnetic spectrum, which is defined in terms of wavelength and frequency. The spectrum of visible light occupies a relatively small range of frequencies between infrared and ultraviolet light (Figure (PageIndex<6>)).

Figure (PageIndex<6>): The electromagnetic spectrum ranges from high-frequency gamma rays to low-frequency radio waves. Visible light is the relatively small range of electromagnetic frequencies that can be sensed by the human eye. On the electromagnetic spectrum, visible light falls between ultraviolet and infrared light. (credit: modification of work by Johannes Ahlmann).

Whereas wavelength represents the distance between adjacent peaks of a light wave, frequency, in a simplified definition, represents the rate of oscillation. Waves with higher frequencies have shorter wavelengths and, therefore, have more oscillations per unit time than lower-frequency waves. Higher-frequency waves also contain more energy than lower-frequency waves. This energy is delivered as elementary particles called photons. Higher-frequency waves deliver more energetic photons than lower-frequency waves.

Photons with different energies interact differently with the retina. In the spectrum of visible light, each color corresponds to a particular frequency and wavelength (Figure (PageIndex<6>)).The lowest frequency of visible light appears as the color red, whereas the highest appears as the color violet. When the retina receives visible light of many different frequencies, we perceive this as white light. However, white light can be separated into its component colors using refraction. If we pass white light through a prism, different colors will be refracted in different directions, creating a rainbow-like spectrum on a screen behind the prism. This separation of colors is called dispersion, and it occurs because, for a given material, the refractive index is different for different frequencies of light.

Certain materials can refract nonvisible forms of EMR and, in effect, transform them into visible light. Certain fluorescent dyes, for instance, absorb ultraviolet or blue light and then use the energy to emit photons of a different color, giving off light rather than simply vibrating. This occurs because the energy absorption causes electrons to jump to higher energy states, after which they then almost immediately fall back down to their ground states, emitting specific amounts of energy as photons. Not all of the energy is emitted in a given photon, so the emitted photons will be of lower energy and, thus, of lower frequency than the absorbed ones. Thus, a dye such as Texas red may be excited by blue light, but emit red light or a dye such as fluorescein isothiocyanate (FITC) may absorb (invisible) high-energy ultraviolet light and emit green light (Figure (PageIndex<7>)). In some materials, the photons may be emitted following a delay after absorption in this case, the process is called phosphorescence. Glow-in-the-dark plastic works by using phosphorescent material.

Figure (PageIndex<7>): The fluorescent dyes absorbed by these bovine pulmonary artery endothelial cells emit brilliant colors when excited by ultraviolet light under a fluorescence microscope. Various cell structures absorb different dyes. The nuclei are stained blue with 4&rsquo,6-diamidino-2-phenylindole (DAPI) microtubles are marked green by an antibody bound to FITC and actin filaments are labeled red with phalloidin bound to tetramethylrhodamine (TRITC).

  1. Which has a higher frequency: red light or green light?
  2. Explain why dispersion occurs when white light passes through a prism.
  3. Why do fluorescent dyes emit a different color of light than they absorb?

CBSE Class 10 Science Chapter 11 Notes Human Eye and Colourful World

Human Eye and Colourful World Class 10 Notes Understanding the Lesson

1. The human eye: The human eye an extremely valuable and a sensitive sense organ, which enables us to see objects and colours around us.

  • Cornea: A thin membrane through which light enters the eye, maximum refraction occurs at the outer surface of cornea.
  • Iris: A dark muscular membrane which controls size of pupil.
  • Pupil: Regulates and controls the amount of light entering the eye.
  • Eye lens: Composed of fibrous, jelly-like material, with adjustable curvature, forms an inverted and real image of object on retina.
  • Retina: It is a light sensitive screen on which image is formed.

  • The ability of the eye lens to adjust its focal length is called accommodation.
  • Least distance of distinct vision: Minimum distance at which object can be seen distinctly without any strain from normal eye, i.e, 25 cm for normal vision.
  • Far point of the eye: The farthest point upto which the eye can see objects clearly is called far point of the eye. It is infinity for normal eye.

4. Defects of Vision:
(i) Cataract: Crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract. It is possible to restore vision through cataract surgery.
(ii) Myopia: (Near sightedness)

A person with myopia can see nearby objects clearly but cannot see distant objects clearly. Cause

(iii) Hypermetropia (far-sightedness)
A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.
Cause

Correction
Convex lens of suitable power.

(iv) Presbyopia
The power of accommodation of the eye usually decreases with ageing. In this eye defect it is difficult to see nearby objects comfortably and distinctly without corrective eye glasses.

Cause:Weakening of cilary muscles and diminishing flexibility of eye lens.

Correction: By using bifocal lens. Upper portion consists of concave lens and lower part is convex lens.

5. Refraction of Light through Prism

(i) The refraction of light takes place at two surfaces firstly when light enters from air to prism and j secondly when light emerges from prism.
(ii) Angle of prism: The angle between the two lateral faces of the prism is called angle of prism.
(iii) Angle of deviation: The angle between incident ray (produced forward) and emergent ray I (produced backward).

6. Dispersion of White Light by a Glass Prism
Dispersion:

  • The splitting of light into its component colours is called dispersion.
  • Red light bends the least while violet bends the most.

Spectrum: The band of the coloured components of a light beam is called spectrum, i.e., VIBGYOR

When an inverted prism is kept a little distance away from the prism causing dispersion or basically in the path of splitted beam, the spectrum recombines to form white light.

7. Rainbow Formation
A rainbow is a natural spectrum appearing in the sky after rain shower. It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. The water droplet act like small prism. They refract and disperse the incident sunlight, then reflect it internally and finally refract it again.

Due to dispersion of light and internal reflection different colours appears.

8. Atmospheric Refraction
If physical conditions of the refracting medium (air) are not stationary, the apparent position of the object fluctuates.

  • The twinkling of stars is due to atmospheric refraction of starlight.
  • When starlight enters the earth’s atmosphere, it suffers refraction continuously. Since the physical conditions of the earth’s atmosphere are not stationary the stars appear twinkling.

Advance sunrise and delayed sunset

Advance sunrise and delayed sunset is due to atmospheric refraction.
When the sun is slightly below the horizon, the sunlight coming from the less dense (vacuum) to the more dense (air) medium is refracted downwards. Therefore the Sun appears to be above the horizon. Similarly, even after sunset, the Sun can be seen for sometime due to refraction of sunlight.

The phenomenon of scattering of light by colloidal particle gives rise to Tyndall effect.
Tyndall effect can be observed when sunlight passes through a canopy of a dense forest. Here tiny droplets in mist scatters light.
The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths.

Colour of the clear sky is blue: The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour more strongly than red.

Danger signal lights are red in colour: Because red colour is least scattered by fog or smoke.

Sun appears reddish early in the morning: In the morning and evening, the Sun lies near the horizon. Sunlight travels through a larger distance in the atmosphere and most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelength. This gives rise to the reddish appearance of the Sun.

Class 10 Science Chapter 11 Notes Important Terms

Eye: The human eye is an extremely valuable and sensitive sense organ, which enables us to see objects and colours around us.

Power of accommodation: The ability of the eye lens to adjust its focal length is called accommodation.

Myopia: A person with myopia can see nearby objects clearly but cannot see distant objects clearly.

Cataract: Crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract.

Hypermetropia: A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.

Presbyopia: The power of accommodation of the eye usually decreases with ageing. In this eye defect, it is difficult to see nearby objects comfortably and distinctly without corrective eye glasses.

Dispersion: The splitting of light into its component colours is called dispersion.

Atmospheric refraction: Refraction of light by the constituent particles of the atmosphere. Tyndall effect: The phenomenon of scattering of light by colloidal particles gives rise to Tyndall effect.


Electronic Contact Lenses Put Data on Your Eye

We’ve seen it thousands of times in movies massive amounts of information unspooling right before someone’s eyes, without the need for any type of monitor. Now, fiction is closer to becoming fact as a working model of electronic contact lenses proves to be successful with rabbits.

The current incarnation of the lens isn’t something that would excite anyone with those Hollywood movies in mind because the device only displays one pixel, but it’s the concept behind that one pixel that’s the real attention-getter because where one pixel can go, others can follow. According to PopSci, Professor Babak Parviz says the next step is to “incorporate some predetermined text in the contact lens.”

However, in addition to going beyond one pixel, there are a few other hurdles to overcome. The first problem is power. The current version of the contact lens draws energy from an external source using an antenna that has a range of one meter in free space and only two centimeters when the lens is placed on the eye. The other issue concerns the eye itself. The minimal focal distance of the human eye is a few centimeters so information that would displayed on a contact lens would be blurry. To take care of this particular problem, researchers used thin Fresnel lenses to magnify the display. There’s no information at this time on if the process will be refined at some point or how exactly a magnified display might affect vision when not reading text on a contact lens.

When it comes to limitless amounts of data being streamed directly to the eye, the future is closer but it still has some travelling to do before it gets here.


Science Question World

Ans. The pupil of an eye acts like a variable aperture whose size can be varied with the help of the iris and the adjustment of the pupil takes time. So, when we enter from bright sunlight to a dark room, we cannot see initially.

3. A person uses spectacles of power +2D. What is the defect of vision he is suffering from?
Ans. A person who uses spectacles of power +2D means he is suffering from hypermetropia (long-sightedness).

4. Why do chickens wake up early and sleep early?
Ans. Chickens have a large number of rod cells that help them to detect the intensity of light. Thus, chickens wake up early and go to sleep early .

5. What is the nature of the image formed at retina?
Ans. T he image formed at the retina is diminished, inverted and real.

6. What is the cause of colour blindness?
Ans. C one cells of the retina are sensitive to colours and when these cells do not respond properly , enable the retina to distinguish between colors.

7. State the structure of iris and its functions in the human eye.
Ans. A structure called iris behind the cornea is a dark muscular diaphragm that controls the size of the pupil and the pupil regulates and controls the amount of light.

8. Define the distance of distinct vision and give its range.
Ans. The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision and its range is about 25 cm .

9. What is meant by the least distance of distinct vision?
Ans. T he least distance of distinct vision means the minimum distance, at which objects can be seen most distinctly without strain.

10. Define the power of accommodation of the eye.
Ans. The ability of the eye lens to adjust its focal length is called the power of accommodation.

11. Why the clear sky appear blue?
Ans. When sunlight passes through the atmosphere the fine particles in the air scatter the blue colour, so the clear sky appears blue.

12. Why does it take some time to see objects in a cinema hall when we just entered the hall from bright sunlight? Explain in brief.
Ans. The pupil of an eye acts like a variable aperture whose size can be varied with the help of the iris and the adjustment of the pupil takes time. So, it takes some time to see objects in a cinema hall when we just entered the hall from bright sunlight.

13. How does the thickness of the eye lens change when we shift looking from a distance tree to reading a book?
Ans. The thickness of the eye lens increases when we shift looking from a distance tree to reading a book.

14. A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. W hat advice will a doctor give to her?
Ans. The student is a short-sightedness or Myopia and a doctor will give her advice to take a spectacle of -ve power means the concave lens of suitable power.
.
15. A hyper meteoric person prefers to remove his spectacles while driving. Give reason.
Ans. A person with hypermetropia can see distant objects clearly and during driving a person has to see more than a nearer point (25 cm). This is because
a hyper meteoric person prefers to remove his spectacles while driving.

16. How are we able to see nearby and also the distant objects clearly?
Ans. W e are able to see nearby and also the distant objects clearly by the ability of the eye lens to adjust its focal length that is called power accommodation.

17. Why do parallel rays of different colours deviate differently while passing through a glass prism?
Ans. Different colours of light bend through different angles with respect to the incident ray while passing through a prism as they have different wavelengths.

18. Name any two phenomena associated with the formation of the rainbow.
Ans. T wo phenomena associated with the formation of the rainbow are internal reflection and dispersion.

19 . Draw a ray diagram showing the dispersion through a Prism when a narrow beam of white light is incident on one of its refracting surfaces. Also, indicate the order of the colours of the spectrum obtained.
Ans. Ans.

20. Define the angle of deviation.
Ans. The angle between the incident ray and emergent ray is called the angle of deviation.

21. List the colours into which light splits in the decreasing order of their bending on emergence from the prism.
Ans. Red, orange, yellow, green, blue, indigo and violet.

22. A beam of white light splits when it passes through a Prism. Name this phenomenon and give its reason.
Ans. The phenomenon is refraction and the reason is the different wavelengths of a different colour and different colour deviate from different angles.

23. Why does the sun look reddish at the time of sunrise and sunset? Explain.
Ans. During sunrise and sunset , l ight from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere. Shorter wavelengths are scattered away by the particles and most of the red light of a longer wavelength which is least scattered reaches our eyes. This gives rise to the reddish appearance of the Sun.

24. Why do different components of white light split up into a spectrum, when it passes through a triangular glass prism?
Ans. Different colours of light bend through different angles with respect to the incident ray while passing through a prism as different colours have different wavelengths so deviate from different angles.

25. What is the dispersion?
Ans. The splitting of light into its seven component colours is called dispersion.

26. What happens when light is passed through a glass prism.
Ans. Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism.

27. What is astigmatism?
Ans. Astigmatism is a common vision problem caused by irregular-shaped of cornea, that causes blurred vision.

28. Name the defect of vision in which the eye loses its power of accommodation due to old age.
Ans. Presbyopia.

II. Short answer type questions:

(b) State two reasons due to which the myopia eye defect may be caused?
Ans. (a)

(b) This defect may arise due to
(i) excessive curvature of the eye lens, or
(ii) elongation of the eyeball.

Ans. No, the position of a star as seen by us is not it's true position. The atmospheric refraction occurs in a medium of gradually changing the refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher than its actual position when viewed near the horizon.

16. What will be the colour of the sky be for an astronaut staying in the International Space Station orbiting the earth? Justify your answer by giving reasons.

Ans. T he colour of the sky will be black for an astronaut staying in the International Space Station orbiting the earth because there is no atmosphere in the space and the light reaching it does not scatter. Scattering of blue light of short wavelength causes the blue colour of the sky.

Ans. The angle between the incident ray and emergent ray is called the angle of deviation.
Different components of white light split up into spectrum when it passes through a triangular glass prism because different colour has a different wavelength and deviate with different angles.

24. Why the power of accommodation of an eye decreases with age? Explain.
Ans. The power of accommodation of the eye usually decreases with ageing. I t arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.

25. Draw ray diagram each show:
(i) Myopic eye (ii) Hypermetropic eye.
Ans. (i) Myopic eye-

III. Long answer type questions:

1. A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain.
Ans. Myopia is known as short-sightedness. A myopic person can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.
This defect may arise due to
(i) excessive curvature of the eye lens, or (ii) elongation of the eyeball.
This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.


2. (i) Why the student is unable to see distinctly the objects placed beyond 5m from his eyes.
(ii) the type of corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.
Ans. See the answer of Q.1

3. List the parts of the human eye that control the amount of light entering into it. Explain how they perform this function.
Ans. Iris and pupil are the two parts of the eye that controls the amount of light entering into it. Iris behind the cornea is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye.
The pupil of an eye acts like a variable aperture whose size can be varied with the help of the iris. When the light is very bright, the iris contracts the pupil to allow less light to enter the eye. However, in dim light, the iris expands the pupil to allow more light to enter the eye. Thus, the pupil opens completely through the relaxation of the iris.

4. Write the function of the retina in the human eye. Do you know that corneal impairment can be cured by replacing the defective cornea with the cornea of a donated eye? How and why should we organise groups to motivate the community members to donate their eyes after death?
Ans. The retina of human eye act as a screen. The eye lens forms an inverted real image of the object on the retina. The retina is a delicate membrane with having an enormous number of light-sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.
By donating our eyes after we die, we can light the life of a blind person.
About 35 million people in the developing world are blind and most of them can be cured. About 4.5 million people with corneal blindness can be cured through corneal transplantation of donated eyes. One pair of eyes gives vision to TWO CORNEAL BLIND PEOPLE.

5. List three common refractive defects of vision. Suggest the way of correcting these defects.
Ans. Three common refractive defects of vision are myopia or short-sightedness, hypermetropia or long-sightedness and presbyopia.
Myopia - A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina. This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
Hypermetropia - Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant object clearly but cannot see nearby objects distinctly. This is because the light rays from a close-by object are focussed at a point behind the retina.
This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
Presbyopia- The power of accommodation of the eye usually decreases with ageing. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Such people require A common type of bi-focal lenses consists of both concave and convex lenses. These days, it is possible to correct the refractive defects with contact lenses or through surgical interventions.

6. About 45 lakh people in the developing countries are suffering from corneal blindness about 3 lakh children below the age of 12 suffering from this defect can be cured by replacing the defective, with the cornea of a donated eye. How and why can a student of your age involve themselves to create awareness about this fact among people?
Ans. Try to yourself.

7. A person cannot read a newspaper place near 50 cm from his eye. Name the defect of vision he is suffering from? Draw a ray diagram to illustrate the defects. List two possible causes. Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length. We see an advertisement for eye donation on television or a newspaper. Write the importance of such advertisement.
Ans. A person cannot read a newspaper place near 50 cm from his eye. The person is hypermetropic. He can see distant objects clearly but cannot see nearby objects distinctly. This is because the light rays from a close-by object are focussed at a point behind the retina . This defect arises either because
(i) the focal length of the eye lens is too long, or
(ii) the eyeball has become too small.
This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.



Advertisement for eye donation on television or a newspaper helps to blind people around us and more people can aware of this noble cause.

8. (a) What type of spectacles should be worn by a person having the defect of myopia as well as hypermetropia.
(b) The far point of a myopic person is 150 cm. What is the nature and the power of the lens required to correct the defect?
(c) With the help of a ray, a diagram showing the formation of image by:
(i) a myopic eye
(ii) Correction of myopia by using an appropriate lens.
Ans. (a) Spectacles of the concave lens or diverging lens should be worn by a person having the defect of myopia and converging lens for hypermetropia.
(b) The far point of a myopic person is 150 cm. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this defect has a far point nearer than infinity. A concave lens or diverging lens of suitable - ve power will bring the image back on to the retina and thus the defect is corrected.

9. A person's image when seen through a stream of hot air rising above a fire disappeared to waver. Explain.
Ans. The apparent random wavering of objects seen through a stream of hot air rising above a fire or a radiator because the air just above the fire becomes hotter than the air further up.
The hotter air is lighter or less dense than the cooler air above it, and has a refractive index slightly less than that of the cooler air. Since the physical conditions of the refracting medium are not stationary, the apparent position of the object, as seen through the hot air, fluctuates. This wavering is thus an effect of atmospheric refraction on a small scale in our local environment.

10. (a) Describe an activity along with a level diagram of the phenomenon of dispersion through a Prism.
(b) Explain in brief the formation of the rainbow with the help of the figure.
Ans. (a) Activity - Take a thick sheet of cardboard and make a small hole or narrow slit in its middle.
Allow sunlight to fall on the narrow slit. This gives a narrow beam of white light.
Now, take a glass prism and allow the light from the slit to fall on one of its faces.
Turn the prism slowly until the light that comes out of it appears on a nearby screen.
We will find a beautiful band of colours due to the dispersion of light.










Activity: . Place a strong source (S) of white light at the focus of a converging lens (L1). that provides a parallel beam of light.
. Allow the light beam to pass through a transparent glass tank (T) containing clear water.
. Allow the beam of light to pass through a circular hole (c) made in cardboard. Obtain a sharp image of the circular hole on a screen (MN) using a second converging lens (L2).
.Dissolve about 200 g of sodium thiosulphate in about 2 L of clean water taken in the tank.
Add about 1 to 2 mL of concentrated sulphuric acid to the water.
We can observe the blue light from the three sides of the glass tank that is due to scattering of short sulphur particles. The colour of the transmitted light from the fourth side of the glass tank facing the circular hole, at first the orange red colour and then bright crimson red colour on the screen.
Two chemicals used in this activity are sodium thiosulphate and sulphuric acid .

12. (I) Define dispersion. How does a prism disappear white light? Which colour of light bends the most and the least?
(II) A narrow beam of white light is passing through a glass prism. Trace it on your answer sheet and show the path of the emergent beam as observed on the screen.
(a) Write the name and the cause of the phenomenon observed.
(b) Where else in nature in this phenomenon observed.
(c) Base on the observation, state the conclusions which can be drawn about the constitution of white light.
Ans. (I) The splitting of light into its component colours is called dispersion.
White light is dispersed into its seven-colour components by a prism. Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. It is due to different wavelengths of different colour.
The red light bends the least while the violet the most.
(II)

(a) The phenomenon is a dispersion of light and it caused due to different colours of light bend through different angles with respect to the incident ray as they have different wavelengths.
(b) Rainbow after rain.
(c) The prism has probably split the incident white light into a band of seven colours. The sequence of colours are Violet, Indigo, Blue, Green, Yellow, Orange, and Red. (VIBGYOR)

13. State the natural phenomenon behind the formation of the rainbow? Explain the phenomenon. Name a device that can be used to observe such a phenomenon in the laboratory? If you are facing a rainbow in the sky, what is the position of the sun with respect to your position?
Ans. The natural phenomenon behind the formation of rainbow is dispersion.
The splitting of light into its component colours is called dispersion.
White light is dispersed into its seven-colour components by a prism. Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. It is due to different wavelengths of different colour.
The red light bends the least while the violet the most.
Prism is used to observe dispersion in the laboratory.
A rainbow is always formed in a direction opposite to that of the Sun. Therefore the position of the Sun behind me.

14. An old person is unable to see clearly nearby objects as well as distinct objects.
(a) What defect of vision is the suffering from?
(b) What kind of lens will be required to see clearly the nearby as well as distant objects? Give reasons.
Ans. (a) The defect of vision is Presbyopia in which he finds it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia.
(b) A common type of bi-focal lenses consists of both concave and convex lenses.
Reason: It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. So a person may suffer from both myopia and hypermetropia. A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates a distant vision. The lower part is a convex lens. It facilitates near vision.


A Change in Worldview: Vision Correction Explained

Nearly 75% of the American population relies on some form of visual aid. According to The Vision Council, approximately 64% of people wear eyeglasses, while 11% rely on contact lenses. For most of us, these statistics are no surprise ⁠— visual aids have been a prevalent part of our society for centuries. Many of us acquired our first pair of glasses or contacts at a young age, developing optical reliance early on.

But what really are these instruments, and why do so many of us need them? How do our lenses transform the way we see the world? Before delving into such questions, it is helpful to understand the physiology of sight .

Anatomy of the human eye

When light enters the cornea, the clear outermost layer of the eye, it is bent towards the pupil, the opening of the eye. The pupil constricts and dilates in accordance with the environment through a process called pupillary light reflex: in dim settings, the pupil expands for better visual detection, while in brighter settings, it constricts to moderate light exposure. The transmitted light then passes through the lens and is bent once more before extending to the retina. This double-bending mechanism flips all visual input however, the brain reverts it right-side up before cognitive perception occurs. To reach the brain, visual images are coded into electrical impulses that travel along the optic nerve, eventually making their way to the occipital lobe of the cerebral cortex.

Normally, light that enters the lens is fixated on a precise location in the retina known as the focal point. The quality of this fixation is dependent on the distance between the lens to the retina. When this distance deviates from the ideal length, the focal point forms either in front of or behind the retina rather than on the retina itself. The result is an imprecise scattering of light termed refractive error . Refractive errors disturb the clarity of sight by diminishing how visual input is focused and eventually interpreted. Because the shape and size of the human eye continue to change throughout development and adulthood, visual complications can occur at nearly any point in life.

The most prevalent refractive condition is myopia, commonly referred to as nearsightedness. In myopia, the axial length (distance between the lens and retina) is abnormally elongated such that the focal point occurs before the retina. This condition, resulting in a decreased ability to see distant objects clearly, affects an estimated 25% of Americans .

Myopia (top) and hyperopia (bottom), illustrating their respective focal point errors forming before and beyond the retina.

Alarmingly, the frequency of nearsightedness has doubled in the United States since 1971. In East Asian countries such as China, Taiwan and Japan, the prevalence of myopia in young adults approximates 70- 90% . This finding indicates that environmental influence on vision is more prominent than previously believed, as children in these countries are reported to spend more time indoors and away from sunlight. In fact, one Taiwanese study found that light intervention significantly decreased myopic shift and axial elongation in schoolchildren who spent at least 11 hours a week outdoors for a one year duration . Another study found that students who played outdoor sports showed the least potential for myopic development .

The theorized mechanism for this effect is that dopamine secretion in the retina, which is induced by light, inversely correlates to axial elongation . Though the concise role of dopamine in this process is still uncertain, one explanation states that retinal dopamine agonists, which are substances that initiate a physiological response once bound to receptors, “interact with the early signaling molecule ZENK.” The result is an initiation of postnatal eye growth .

The severity of myopia in patients can be graded as mild, moderate, or high, depending on the extent of optical power needed for correction. Whereas mild myopia is most common and easily managed, high myopia is associated with more serious conditions, including retinal damage, glaucoma, and cataracts. Glaucoma, which results in damage to the optic nerve, and cataracts, which disturb visual clarity due to protein accumulation in the lens, are progressive pathologies. In severe cases, they can result in complete vision loss.

In contrast to myopia, hyperopia (also known as farsightedness) is marked by a shortened axial length, resulting in the focal point forming beyond the retina. Consequently, farsighted individuals are able to see distant objects clearly but report blurred vision at closer distances. Hyperopia is present in 10% of individuals in the United States and, like myopia, is diagnosed through a refractive assessment. Hyperopia also shows an association with the environment: a 2008 study conducted in Poland discovered that hyperopia presents at a lower frequency among schoolchildren raised in the city compared to those living in the countryside. This ailment has also shown to worsen with age due to a gradual increase in the rigidity of the lens over time. When farsightedness emerges in later adulthood, it is defined as presbyopia .

Hyperopia can be clinically diagnosed in several ways, including as simple or pathological. Simple hyperopia corresponds to the refractive error caused by axial shortening, while pathological hyperopia is attributed to “[prenatal, neurological, or inflammatory] maldevelopment, ocular disease, or trauma .”

Though most refractive errors are congenital , meaning present at birth , they can worsen throughout development. A primary reason for this progression is that ocular tissue continues to grow before and during adulthood. Consequently, t he progression of myopia is often inevitable. In contrast, the shortened axial length seen in hyperopia may be naturally corrected over time due to this growth ⁠— a process termed accommodation .

The optical treatment for myopia (top) as illustrated by the placement of a minus lens, altering light refraction at the cornea.

Because the causes of myopia and hyperopia are related to the refraction of light, their treatment directly involves the modification of such refraction. The treatment for all forms of optical error involves refractive modification through corrective lenses such as eyeglasses and contact lenses. More specifically, myopia is corrected by the diversion of light through a minus lens, which consists of a thick base and thin center. This structure promotes the focus of light at the retina. In contrast, hyperopia is corrected through a plus powered lens, which is composed of a thicker center that shifts the focal point forward. Though the use of glasses and contacts are increasingly prevalent, the development of such optics date back to as far as the 13th century.

The first pair of spectacles is believed to have emerged in Pisa, Italy, though the conceptualization of optical aids existed much earlier. During the Middle Ages, for instance, scholars looked through glasses filled with water as a means of magnifying scripture. Eventually, single-lens frames composed of glass were hand-held to enhance near-sighted reading. By the late 1200s, magnifying lenses were doubled and connected along the nose bridge by various materials — think leather, wood, metal, or even animal bones — paving the way to the modern eyeglass.

A silver lined frame crafted by Carl Fredrik Jonssén in 1850. (Depiction of Source )

Centuries later, contact lenses made their appearance through a series of flawed, yet increasingly efficient, introductions . In 1801, a young English scientist by the name of Thomas Young was inspired by Renee Descartes’ innovative notion that optical aids could be worn in direct contact with the lens of the eye. Young designed a thin glass tube containing water (for magnifying purposes) and applied the tube to his eyes using a wax adhesive. In retrospect, this was both dangerous and inefficient at the time, however, Young paved the way for centuries of contact lens development.

Today, many advancements in contact lenses have allowed for increased comfort and safety. Most lenses provide moisture for longer wear-time following the emergence of hydrogel plastics. In addition, they can be worn either during the day or overnight, with the latter providing temporary day-time correction (known as “corneal reshaping contact lenses”).

In addition to the advancements in both eyeglasses and contact lenses, newer and more permanent technologies have emerged within the last several decades. Refractive eye surgery, also known as LASIK (laser-assisted in situ keratomileusis) involves the use of a pulsating laser beam for precise reshaping of the cornea. The process is relatively brief and begins with the incision of a thin flap of the cornea to expose underlying tissue curvature, followed by numbing eye drops and ocular tissue removal. In cases of myopia, the cornea is shaped in a concave manner to promote a long-lasting reduction in refractive power. With hyperopia, tissue is flattened along a spherical circumference to produce a sharper convex shape. With high rates of success and a low probability for complications, LASIK is best suited for individuals with mild to moderate myopia or hyperopia. It can also treat astigmatism, which is an impairment in vision characterized by imperfect curvature of the cornea. Surgical treatment is not suitable for severe myopia, as it would require too large a fraction of tissue removal.

Despite the increasing global prevalence of refractive errors, many misconceptions continue to surround what does or does not worsen eyesight. While natural light exposure does play a role in the progression of myopia and hyperopia, most lifestyle patterns do not. Squinting, for example, may be indicative of myopia but does not affect its progression. Similarly, while extended screen time may cause temporary eye discomfort (termed “digital eye strain”), studies show a limited impact of blue light on long-term visual impairment .

Another common misconception is that individuals who wear corrective lenses develop a physiological reliance on them or weaken their eyes through their usage. While refractive errors can worsen over time, the changes in our prescriptions are not a result of the optical aids we wear.

Finally, a prevailing household misconception is that carrots are beneficial to vision many of us can likely recall being given cups of carrot juice to “strengthen our eyes.” While high in Vitamin A, carrot juice provides minimal, if any, impact on refractive errors. Nonetheless, the idea that vegetables can improve eye health has, in fact, been supported empirically. Leafy greens containing lutein and zeaxanthin carotenoid pigments have shown to prevent eye diseases such as age-related macular degeneration, which results in damage to the retina. Such pigments reduce the amount of light-induced oxidation in the retina, a process associated with harmful, high-energy blue light.

With the frequency of vision ailments on the rise, it is critical that we understand the causes and management of refractive errors. Though our ability to alter our visual predispositions is limited, there are measures we can take to protect our eyes. The best form of self-care includes regularly visiting an optometrist, spending time outdoors, and maintaining a balanced diet.

“ Glasögon “ by Bohusläns museum is licensed under CC BY-NC-ND 4.0


Focal Length Examples

To find the focal length of a lens, measure the distances and plug the numbers into the focal length formula. Be sure all measurements use the same measurement system.

Example 1​: The measured distance from a lens to the object is 20 centimeters and from the lens to the image is 5 centimeters. Completing the focal length formula yields:

The focal length is therefore 4 centimeters.

Example 2​: The measured distance from a lens to the object is 10 centimeters and the distance from the lens to the image is 5 centimeters. The focal length equation shows:


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