A cataract is a progressive, painless clouding of the natural, internal lens of the eye. It begins when proteins in the eye form clumps that prevent the lens from sending clear images to the retina. The retina works by converting the light that comes through the lens into signals. It sends the signals to the optic nerve, which carries them to the brain. For people who have cataracts, seeing through cloudy lenses is a bit like looking through a frosty or fogged-up window.