The eye and vision

Vision is the sense by which the eye detects light and forms images, and the eye is often compared to a camera. Light entering the eye is focused onto a light-sensitive surface, converted into nerve signals, and sent to the brain for interpretation. Animations are well suited to showing how rays of light are bent and brought to a focus inside the eye, a path that is otherwise hard to visualise.

Light first passes through the transparent cornea at the front of the eye, which does most of the bending of the rays. It then travels through the pupil, the opening whose size is adjusted by the coloured iris to control how much light enters. Behind the pupil, the lens fine-tunes the focus, changing shape to bring near or distant objects into sharp view, a process called accommodation. The focused light lands on the retina at the back of the eye.

The retina contains two kinds of light-sensitive cells: rods, which are very sensitive and work in dim light but do not distinguish colour, and cones, which work in bright light and provide colour vision. These cells convert light into electrical impulses, which travel along the optic nerve to the brain, where the image is interpreted. Because the lens forms an inverted image on the retina, the brain effectively turns the picture the right way up. Common vision problems such as short-sightedness and long-sightedness arise when the image is focused slightly in front of or behind the retina.

Frequently asked questions

How does the eye focus light?

The cornea does most of the focusing, and the lens fine-tunes it by changing shape to bring near or distant objects sharply onto the retina.

What is the difference between rods and cones?

Rods are sensitive in dim light but cannot see colour; cones work in bright light and give colour vision. Both are found in the retina.

What does the iris do?

The iris is the coloured ring that adjusts the size of the pupil, controlling how much light enters the eye.