Anatomy of the human eye

The eye is the organ of sight, a roughly spherical structure that forms an image of the outside world and converts it into nerve signals. Light first passes through the cornea, the clear curved front surface that does most of the eye's focusing. Behind it lies a chamber filled with a watery fluid called the aqueous humour, which helps maintain the eye's shape and pressure. The white outer coat surrounding the rest of the eyeball is the sclera.

The coloured part visible through the cornea is the iris, a ring of muscle that controls the size of the central opening, the pupil. By widening or narrowing the pupil, the iris regulates how much light enters the eye. Just behind the iris sits the lens, a transparent, flexible structure held in place by fine fibres called zonules attached to the ciliary body. By changing the lens's shape, the ciliary muscle adjusts focus between near and distant objects, a process known as accommodation.

Light focused by the cornea and lens crosses the large rear chamber, which is filled with a clear gel called the vitreous humour, and falls on the retina lining the back of the eye. The retina contains light-sensitive cells: rods, which work in dim light, and cones, which provide colour vision and detail. A small central region of the retina, the macula, gives the sharpest vision, while the surrounding retina handles peripheral sight. The cells convert light into nerve impulses that gather and leave the eye through the optic nerve, which carries the visual information to the brain.

Frequently asked questions

What does the iris do?

The iris is the coloured muscular ring that adjusts the size of the pupil, controlling how much light enters the eye in bright or dim conditions.

How does the eye change focus?

The ciliary muscle changes the shape of the lens, making it thicker for near objects and thinner for distant ones, in a process called accommodation.

What is the difference between rods and cones?

Rods are sensitive to dim light and give black-and-white vision, while cones work in brighter light and provide colour vision and fine detail.