Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Cataract ?
    ### Response:
    The lens is a clear part of the eye that helps to focus light, or an image, on the retina. The retina is the light-sensitive tissue at the back of the eye. In a normal eye, light passes through the transparent lens to the retina. Once it reaches the retina, light is changed into nerve signals that are sent to the brain.
    