Physics of the Eye

The eye acts as a camera, refracting light to focus an image on sensory cells to produce neural responses from which an image is constructed in our brain, thus enabling us to see. Your eyeball possesses a transparent opening on the front called the cornea. The cornea is shaped like a converging lens (meaning that it is thicker in the middle). Therefore, it converges rays of light traveling parallel to a single point, optimally on the back of the eye, or retina. The cornea also refracts light thanks to its index of refraction of 1.38, significantly greater than that of air. Light next passes through the pupil, which is basically just an opening. The pupil appears black because all of the light behind it is absorbed by the sensory cells of the retina, such that no light is reflected out of the eye. The colored part of the eye, or iris, is a muscular ring that adjusts the size of the pupil depending on how much light is available. Light then passes through the crystalline lens, a membrane capable of changing shape with the help of ciliary muscles as a mechanism to focus the image on the retina, where all the sensory cells are located. This muscle action also serves to make slight changes in the bulge-shape of the cornea changing the focal length. While the cornea does most of the refracting, the role of the lens is to make small alterations thanks to its flexibility. Generally, the focal length (or length from the lens to the point at which the light rays converge) is approximately 1.8 cm. The anatomy of the eye ensures that at this length, the image produced is focused on the retina in a reduced, inverted form (the brain reverts the image such that we perceive everything right side up). The eye adjusts its focal length via the mechanisms described earlier in a process called accommodation, which allows us to focus on images both close up and far away. Close up objects require a shorter focal length so that the image will focus on the retina. Conversely, far objects require a longer focal length. This means that for distant objects, the ciliary muscles relax, and contract for close objects. To apply this further, nearsightedness (or the inability to focus on objects far away) is caused by a bulging cornea or elongated eyeball which causes a shortening of the focal length. Corrective lenses are shaped divergently (concave in the middle) to spread out the light rays before they enter the eye, thus moving the focal point back to the retina. Farsightedness is the exact opposite; the focal length is lengthened such that convergent lenses are necessary. - written by Keith Casey