When you look at something, light comes into your eye through the hole in the centre (the pupil) and passes through the lens to the back of the eyeball (the retina).The light is then changed into nerve signals that travel to your brain. Your brain tells you what the image is.

If the lens is cloudy, the image on the back of the eye will be blurred. It's rather like looking through a dirty windscreen or frosted glass.Different parts of the lens can become cloudy. Commonly cataracts begin as clouding around the edges of the lens.

You may notice that you get a lot of glare or a ?halo? effect around bright lights, or when the sun is low in the sky. If you drive you might find oncoming headlights more difficult to cope with than before. Cataracts can also affect the middle of the lens, which makes your vision foggy.As the cataract grows:your sight becomes blurry;you may have double vision;you may find it hard to tell the difference between some colours, especially shades of blue;you may see ?halo? effects around lights, especially at night.

Cataracts are common. About a third of people aged over 65 have cataracts in one or both eyes.Getting older and exposure to bright sunlight are the main reasons people get cataracts. Having diabetes, smoking and drinking too much alcohol may also increase your risk of cataracts.

A cataract operation can:help you see better and stop your poor sight interfering with your life. Not everyone with a cataract needs an operation. Doctors usually suggest the operation when your cataract interferes with your daily life rather than when your eyesight reaches a particular score on a sight test.Your cataract may mean you can't see well enough to read, work, play a sport, go shopping or drive a car.

Some people manage with their cataract for longer than others because their poorer sight does not affect them much. For example, a person who loves bird-watching may decide they need treatment before a person whose main hobby is listening to music.In the past, people were advised that their cataract had to be ?ripe? (very bad) before it was worth having an operation.

This is because cataract surgery was a big operation that took a long time to recover from, and the artificial lenses used then were not very good. Now, with safer, quicker operations, cataracts are generally removed earlier, before they cause serious problems. And modern lenses are better.

Sometimes a cataract should be removed even if it does not cause problems with your sight. This is because there is another eye problem that needs treating and the cataract is in the way. Examples of other eye problems are diabetic retinopathy and macular degeneration.They are problems with the blood vessels in the eye.If you have cataracts in both eyes, it's normal to do them one eye at a time, usually a couple of months or more apart. This is because the treated eye can be sore and needs to settle down for a while.

A cataract operation usually lasts about 10 minutes to 20 minutes. You should be able to go home the same day. You may know someone who has stayed overnight after their cataract operation. People usually only stay in hospital if they have problems after their operation, if they have other eye diseases (glaucoma, for example) or other medical problems that doctors need to check.

For most people it's as safe to be at home as it is to stay in hospital, and most people prefer to be in familiar surroundings.At hospitals that are very experienced in doing cataract surgery, you may be in and out of the hospital within 90 minutes.