Light - The Eye

The front of the eyeball contains the lens. This is a flexible sac filled with a jelly-like substance. The walls of the sac are flexible and bulge outwards at the centre so it is a bi-convex lens. This focuses the light entering the eye onto the retina. If the eyeball is too long or too short for the power of the lens, short sightedness or long sightedness is the result. If the lens cannot focus light on the retina, or is too fat or too thin, that can also cause short or long sight. These two common eye defects are easily corrected by using glasses which hold lenses of the correct strength in the correct position in front of the eyes. The lens of the eye is held in place by the suspensory ligament and its shape is controlled by the ciliary muscle. The lens becomes less flexible with age.

Covering the lens and protecting it is the transparent membrane that we call the cornea. This is also involved in focusing the light entering the eye but does not change shape. It is a meniscus lens.

The iris is the equivalent of the diaphragm of a camera lens, it controls the amount of light that enters the eye. It is the coloured part of the eye. It is situated between the cornea and lens, close to the lens. The hole through the middle of the iris is called the pupil. The size of the pupil is determined by the iris. When the light is bright, the muscles of the iris relax, reducing the size of the pupil. When the pupil is very small, it protects the retina from being damaged by high light intensities. In low light, the muscles of the iris contract so the pupil becomes dilated, allowing more light into the eye when it gets dark.