The eye is a sense organ containing receptor cells that are sensitive to light. The structure of the eye allows it to carry out its function; important structural features include the:

• Cornea - Transparent covering at the front of the eye that refracts (bends) light

• Iris - Muscle that controls how much light enters the pupil

• Lens - Transparent disc that changes shape to focus light onto the retina

• Retina - Layer of light receptor cells that detect light intensity and colour

• Optic Nerves - Sensory neurone that carries electrical impulses from the eye to the brain

The pupil reflex is an example of a reflex action; its role is to control the light that enters the eye by altering the pupil diameter

• In dim light the pupil dilates in order to allow as much light into the eye as possible

• IIn bright light the pupil constricts in order to prevent too much light entering the eye and damaging the retina

The pupil reflex occurs due to changes in the iris muscles. The iris contains circular muscles and radial muscles

• The circular muscles form circles around the pupil

• The radial muscles radiate outwards from the pupil

The circular and radial muscles of the iris are antagonistic, meaning that they work against each other

Accommodation is the term used to describe the way in which the eye focuses on near or distant objects. During eye accommodation the shape of the lens is changed, altering the extent to which light is refracted; this change is brought about by:

• contraction or relaxation of the ciliary muscles

• adjustment of tension in the suspensory ligaments

Rods and cones are the two types of receptor cell present in the retina of the eye. Rods can detect light at low levels, so play an important role in night vision. Three different types of cones can detect light at three different wavelengths, enabling colour vision

Rods and cones are not distributed evenly across the retina. Rod cells are found all over the retina, with the exception of the blind spot. Cone cells are concentrated in the fovea, the region of the eye onto which light is focused by the process of accommodation. The fovea enables the brain to form sharp, coloured images when light is effectively focused by the eye.