PHGY 210 Lecture Notes - Aqueous Humour, Pupillary Light Reflex, Cornea

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31 Jan 2013
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IMAGE FORMATION BY THE EYE
Cornea, rather than the lens, is the site of most of the refractive power of the eyes
Refraction by the Cornea
Light strikes the cornea and passes from the air into the aqueous humor. Then light rays bend
and converge on the back of the eye (see Fig. 9.7)
Focal distance: the distance from the refractive surface to the point where parallel light rays
converge and depends on the curvature of the cornea (i.e. tight the curve, the shorter the focal
distance).
Diopter: reciprocal of the focal distance in meters. The cornea has a refractive power of 42
diopters which means that parallel light rays striking the corneal surface will be focused 0.024 m
(2.4 cm).
Refractive power depends on the slowing of light at the air-cornea interface. If we replace air
with a medium that passes light at about the same speed as the eye, the refractive power will
eliminated. This is why one’s vision is blurry when you open your eyes underwater.
Accommodation by the Lens
The lens contributes another dozen or so diopters to the formation of a sharp image at a
distance. It is, however, more importantly involved in forming crisp images of objects located
closer than about 9m from the eye.
As objects approach, the light rays originating at a point can no longer be consider to be parallel.
These rays diverge and greater refractive power is required to bring them into focus on the retina.
This focusing power, called accommodation, is provided by changing the shape of the lens (see
Fig. 9.8).
During accommodation, the ciliary muscles contract and swells in. The lens becomes rounder
and thicker, increasing the curvature of the lens surfaces and increases the refractive power.
The ability to accommodate changes with age.
The Pupillary Light Reflex
The pupil continuously adjusts for different ambient light levels.
Pupillary light reflex involves connections between the retina and neurons in the brain
stem that control the muscles that constrict the pupils.
o This reflex is consensual. Shining a light into only one eye cause the constriction of the
pupils of both eyes.
Constriction of the pupil increases the depth of focus.
The Visual Field
It is the points where one can no longer see an object while staring straight ahead (see Fig.
9.9). The left visual field is imaged on the right side of the retina and the right visual field is
imaged on the left side of the retina.
Visual Acuity
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