Parts of the eye

Figure 5.1 shows the physiology of a human eye. The shape is approximately spherical, with a diameter of around $24$mm and only slight variation among people. The cornea is a hard, transparent surface through which light enters and provides the greatest optical power (recall from Section 4.4). The rest of the outer surface of the eye is protected by a hard, white layer called the sclera. Most of the eye interior consists of vitreous humor, which is a transparent, gelatinous mass that allows light rays to penetrate with little distortion or attenuation.

As light rays cross the cornea, they pass through a small chamber containing aqueous humour, which is another transparent, gelatinous mass. After crossing this, rays enter the lens by passing through the pupil. The size of the pupil is controlled by a disc-shaped structure called the iris, which provides an aperture that regulates the amount of light that is allowed to pass. The optical power of the lens is altered by ciliary muscles. After passing through the lens, rays pass through the vitreous humor and strike the retina, which lines more than $180^\circ$ of the inner eye boundary. Since Figure 5.1 shows a 2D cross section, the retina is shaped like an arc; however, keep in mind that it is a 2D surface. Imagine it as a curved counterpart to a visual display. To catch the light from the output pixels, it is lined with photoreceptors, which behave like ``input pixels''. The most important part of the retina is the fovea; the highest visual acuity, which is a measure of the sharpness or clarity of vision, is provided for rays that land on it. The optic disc is a small hole in the retina through which neural pulses are transmitted outside of the eye through the optic nerve. It is on the same side of the fovea as the nose.