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Refractive indices relevant to the eye
Material Index of Refraction
Water 1.33
Air 1.0
Cornea 1.38
Aqueous humor 1.34
Lens 1.41 average (varies throughout the lens, greatest in center)
Vitreous humor 1.34
Ray diagram in the picture shows the internal structure of an eye and a tree that is taken as an object. An inverted image of the tree is formed on retina with the light rays coming from the top and bottom of the tree; converging most at the cornea and upon entering and exiting the lens. The rays coming from top of the tree are labeled one, two, while the bottom rays are labeled three, four. The inverted image of the tree shows rays labeled three, four at the top and one, two at the bottom.
An image is formed on the retina with light rays converging most at the cornea and upon entering and exiting the lens. Rays from the top and bottom of the object are traced and produce an inverted real image on the retina. The distance to the object is drawn smaller than scale.

As noted, the image must fall precisely on the retina to produce clear vision — that is, the image distance d i size 12{d rSub { size 8{i} } } {} must equal the lens-to-retina distance. Because the lens-to-retina distance does not change, the image distance d i size 12{d rSub { size 8{i} } } {} must be the same for objects at all distances. The eye manages this by varying the power (and focal length) of the lens to accommodate for objects at various distances. The process of adjusting the eye’s focal length is called accommodation    . A person with normal (ideal) vision can see objects clearly at distances ranging from 25 cm to essentially infinity. However, although the near point (the shortest distance at which a sharp focus can be obtained) increases with age (becoming meters for some older people), we will consider it to be 25 cm in our treatment here.

[link] shows the accommodation of the eye for distant and near vision. Since light rays from a nearby object can diverge and still enter the eye, the lens must be more converging (more powerful) for close vision than for distant vision. To be more converging, the lens is made thicker by the action of the ciliary muscle surrounding it. The eye is most relaxed when viewing distant objects, one reason that microscopes and telescopes are designed to produce distant images. Vision of very distant objects is called totally relaxed , while close vision is termed accommodated , with the closest vision being fully accommodated .

Two cross-sectional views of eye anatomy are shown. In part a of the figure, parallel rays from distant object are entering the eye and are converging on the retina to produce an inverted image of the tree shown above the principle axis. The interior lens of the eye is relaxed and least rounded, given as P small. Distance of image d i is equal to two centimeters, which is the measure of the distance from lens to retina. Distance of object d o is given as very large. In part b of the figure, rays from a button, which is a nearby object, are shown to diverge as they enter the eye. The interior lens of the eye, P large, converges the rays to form an image at retina, below the principle axis. Distance of image d i is equal to two centimeters, which is the measure of distance from lens to retina. Distance of object d o is given as very small.
Relaxed and accommodated vision for distant and close objects. (a) Light rays from the same point on a distant object must be nearly parallel while entering the eye and more easily converge to produce an image on the retina. (b) Light rays from a nearby object can diverge more and still enter the eye. A more powerful lens is needed to converge them on the retina than if they were parallel.

We will use the thin lens equations to examine image formation by the eye quantitatively. First, note the power of a lens is given as p = 1 / f size 12{p= {1} slash {f} } {} , so we rewrite the thin lens equations as

P = 1 d o + 1 d i


h i h o = d i d o = m.

We understand that d i size 12{d rSub { size 8{i} } } {} must equal the lens-to-retina distance to obtain clear vision, and that normal vision is possible for objects at distances d o = 25 cm to infinity.

Take-home experiment: the pupil

Look at the central transparent area of someone’s eye, the pupil, in normal room light. Estimate the diameter of the pupil. Now turn off the lights and darken the room. After a few minutes turn on the lights and promptly estimate the diameter of the pupil. What happens to the pupil as the eye adjusts to the room light? Explain your observations.

Questions & Answers

does the force in a system result in the energy transfer?
Lebatam Reply
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Anaele Reply
how to prove that Newton's law of universal gravitation F = GmM ______ R²
Kaka Reply
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Olubukola Reply
prove that the centrimental force Fc= M1V² _________ r
Kaka Reply
prove that centripetal force Fc = MV² ______ r
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mitul Reply
griffts bridge derivative
Ganesh Reply
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please explain; when a glass rod is rubbed with silk, it becomes positive and the silk becomes negative- yet both attracts dust. does dust have third types of charge that is attracted to both positive and negative
Timothy Reply
what is a conductor
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dust particles contains both positive and negative charge particles
corona charge can verify
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Ibrahim Reply
remains the temperature
what is frequency
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Sujitha Reply
CT scanners do not detect details smaller than about 0.5 mm. Is this limitation due to the wavelength of x rays? Explain.
hope this helps
what's critical angle
Mahmud Reply
The Critical Angle Derivation So the critical angle is defined as the angle of incidence that provides an angle of refraction of 90-degrees. Make particular note that the critical angle is an angle of incidence value. For the water-air boundary, the critical angle is 48.6-degrees.
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pls who can give the definition of relative density?
the ratio of the density of a substance to the density of a standard, usually water for a liquid or solid, and air for a gas.
What is momentum
aliyu Reply
mass ×velocity
it is the product of mass ×velocity of an object
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Sean Reply
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Practice Key Terms 2

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