What is the cornea?
The cornea is the window at the front of the eye, its main role is to allow light into the eye and to help to focus that light onto the retina at the back of the eye.
The cornea is only half a millimeter thick in the centre, it is mostly made up of a protein called collagen, which is very strong. The fibres of collagen are arranged to allow transmission of light. The normal cornea contributes 2/3rds of the focussing power of the eye, the remainder of the focussing is done by the lens.