A cataract is clouding or opacity of the lens inside the eye. Inside the eye, behind the coloured part (the iris) with a black hole in the middle (the pupil), is the lens. In a normal eye, this lens is clear. It helps focus light rays on to the back of the eye (the retina), which sends messages to the brain allowing us to see. When cataract develops, the lens becomes cloudy and prevents the light rays from passing through.