The Physics of Vision
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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.

dilated vs undilated eyesPupil and Iris

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

eye accomadation 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