Cataracts are changes in clarity (clouding) of the lens in the eye, which interferes with the passage of light into the eye. As the lens gets increasingly cloudy (opaque), less and less light can get through it.
What effect will this have on the signals reaching the brain about what the eye is seeing?
If less light can reach the retina at the back of the eye through the cloudy lens, then the signals reaching the brain will be weaker and less focused. The cataract will gradually impair the quality of the person's vision and result finally in blindness, when light can no longer get through the lens.
In 2002 the World Health Organization estimated that there were 37 million people in the world who were totally blind (unable to see with both eyes), and 161 million people with some sort of visual impairment affecting their sight. Cataracts are the most common cause of blindness worldwide, contributing to 48% of the 37 million people who cannot see at all.