The eye transforms light beams into electric signals. The brain translates these signals so we can see.

In order to achieve a clear image the light beams must be directed in such a way that they come together perfectly on the retina in the back of the eye. First the rays of light are bent by the outer part of the cornea and subsequently by the natural lens. The cornea is the outer part of the eye; it is transparent and it has a "ball form" shape. The rays of light that hit the eye are bend by the cornea.

The natural lens is flexible and can change its form (accommodate). It is located behind the iris and is responsible for focusing the rays of light. Strong deviation of the rays for instance is necessary for reading.
Between the age of 40 and 50 the lens becomes less flexible, this leads to presbyopia.