The opacities in the lens scatter the light, causing hazy vision, in the same way that a dirty window scatters light. Normal eye, with no cataract. Light is focussed on the retina. Eye with cataract. Light is scattered, causing blurred, hazy vision.
Cataracts are due to a change in the lens material.
However cataracts can become worse as more of the lens material changes. Most cataracts are a result of ageing and long-term exposure to ultraviolet light. Some are caused by injury and certain diseases and in rare cases by exposure to toxic materials and radiation. Occasionally cataracts are present at birth, due to the baby's mother having had rubella during the pregnancy, or genetic defects. The clouded areas become larger and denser and cause the patient's sight to become worse.