Rhiannon Schmieg, age 12 of Brown Deer, Wis., for her question:

WHAT IS A CATARACT?

A cataract is the clouding of the lens of the eye. Cataracts usually begin as small spots in the lens that interfere slightly with vision. They can cause blindness by spreading until the entire lens becomes milky white and nontransparent.

The lens keeps the eye in focus. Light rays from an object strike the cornea, the transparent part of the outside of the eyeball. The cornea bends the light rays toward each other, but not enough to focus them into an image. The light rays then pass through the lens, which bends them further and causes them to focus an image on the retina which is located on the back layer of the eyeball.

Because the lens is flexible, it can change shape to help a person focus on objects at different distances. Thus, clear vision depends on light passing through the lens correctly.