Refraction in the Eye
The vision process relies heavily on the
ability of the eye to refract light.
This takes place at both the cornea and the lens
of the eye.
The process of vision first starts with the light
passing through the cornea. Since the cornea has a
spherical surface it acts like a converging lens. as light
rays hit the cornea, the light is refracted. Most
of the refractive power in the eye comes from the
cornea, due to the differences in the indices of
refraction between the air (refractive index
of about 1.00) and the aqueous humor, which has an
index of refraction of 1.34.
Once the light passes through the cornea it
goes through the pupil.This is the aperture of the
eye, so it is essentially a hole that regulates
light intensity by changing the pupil's diameter.
So when its dark, the pupils dilate , up to about
8mm, so that more light can enter the eye. And
when it is bright the pupil shrinks, to an average
of 3mm, to restrict the amount of light.
The pupil's size is controlled by the iris.
Muscles in the the iris cause the pupil to shrink
or dilate. A circular muscle layer controlled by
parasympathetic neurons constrict the pupil, and a
radial muscle layer controlled by sympathetic
neurons dilate the pupil.
The Lens and Accommodation
After light rays travel through the pupil they hit
the lens. Refraction also takes place at this
converging lens of the eye. However since the
vitreous humor has a refractive index of 1.34, and
this lens has a refractive index of 1.44, the
refractive power is not as strong as in the
cornea. This makes the lens responsible for fine
tuning an image that they eye sees.
The eye focuses an image through the process of accommodation,
where the ciliary
muscles can change the curvature of the
lens. When the eye is looking at a distance little
accommodation is needed, so the ciliary muscles
are relaxed, and the lens has the longest focal
length. To see a nearer object, the focal length
of the lens needs to decrease to produce the image
on the retina, to so this the ciliary muscles
contract causing the lens to bend and bulge out.