The eye

Retina - packed with receptors cells which are sensitive to brightness and the colour of light.

Optic nerve - Transmits visual information by electrical impulses from the retina to the brain.

Sclera - white of the eye, the protective outer layer.

Pupil - hole in Iris, which light passes through to get to the retina.

Iris - controls pupil, so control the quantity of light reaching the retina.

Cornea - responsible for refracting light

Ciliary muscle - muscles that change the shape of the lens, which is used to focus light.

Suspensory ligaments - fibres that connect muscles to the lens.

too much light near Iris - Iris decreases in size.

Dim light near Iris - Iris increases in size.

Eyes focus on a distant object - suspensory ligaments tighten, lens become flatter, ciliary muscles relax, minor refraction of light rays.

Eye focus on close object - suspensory ligaments loosen, lens become thicker, ciliary muscles tighten, lots of refraction of light rays.

short-sightedness - rays of light focus in front of the retina.

Long-sightedness - rays of light focus behind the retina.

Cataracts - build-up of protein on the lens that makes the pupil cloudy.

Treating eye defects - glasses filled with lenses, contact lenses and laser eye surgery, synthetic lenses can replace faulty lenses with cataracts.