Focusing of the eye. Animation of a collimate light beam (parallel green lines) entering a mechanical eye and being refocused by the eye lens (light green). At first the lens is focused on infinity, as the human eye is at rest, and the light is focused (narrowest point of beam) on the retina, the light-sensitive membrane at the back of the eye. As the lens thickens, its curvature increases. This increases the len's converging power and the focus of the light beam begins to move towards the front of the eye.