History of glasses Others

Eyes are organs that detect light. Different kinds of light-sensitive organs are found in a variety of animals.
The simplest eyes do nothing but detect whether the surroundings are light or dark, which is sufficient for
the entrainment of circadian rhythms but can hardly be called vision. More complex eyes can distinguish
shapes and colors. The visual fields of some such complex eyes largely overlap, to allow better depth
perception (binocular vision), as in humans; and others are placed so as to minimize the overlap, such
as in rabbits and chameleons.

In the human eye, light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells
called rods (for brightness) and cones (for color) react to the light. They interact with each other and send messages to the brain that indicate brightness, color, and contour.

The first proto-eyes evolved among animals 540 million years ago. Almost all animals have eyes, or descend from animals that did. In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells
(for color) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals.
The visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically roughly
spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and
often an iris; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye, and reducing aberrations
when there is enough light.

The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens—similar to how a camera focuses.