The human eyes work by allowing light to enter through the pupil. The lens is used to focus light rays onto the retina, which is the surface at the back of the eye that is sensitive to light.

The retina consists of two types of receptor cells known as rods and cones. Rods allow us to differentiate between shades of black and white while cones allow us to see colour. Most cones are concentrated in a particularly sensitive region of the retina called the fovea. This region enables us to see the greatest detail. The receptor cells use the optic nerve to transmit signals to the brain, which interprets the signals from both eyes to construct the image we see. The combined image from both eyes assists depth perception.

Common conditions such as short and long-sightedness can occur due to a misshapen lens or eyeball, and can be corrected by glasses. However, as the eye muscles weaken, or the lens becomes stiffer, the eye can no longer adapt to focus on objects at different distances.

Because of ageing and various eye conditions, the structure and function of the eye can also deteriorate, which can result in blurry vision, central or peripheral loss of visual field, and reduced contrast sensitivity. Colour blindness is mainly inherited as a genetic defect, although the ability to distinguish between blue and green can decrease with age.