"The eye works similar like a camera. Light passes through the camera´s curved lens which focuses the image. The image forms behind the lens and is recorded on the film or the digital sensor. In a similar way light passes through the lens of a human eye. It first passes through the cornea, this transparent membrane protects the eye from dust. It also bends the light passing through the eye slightly so that the resulting image will be focused. Light then enters the eye through the pupil, the opening of the eye, and passes through the iris, sheets of smooth muscle, that expand and contract to regulate how much light enters the inside of the eye. In bright light the iris contracts or shrinks around the pupil, this keeps too much light from entering. But when it is dark the iris expands to let as much light as possible into the eye.
Then the eye reaches the lens. The shape of the lens changes constantly to focus an image. Muscles surrounding the lens expand and contract, focusing the light as it travels to the retina located at the back of the eye.
The retina functions in a similar way to the film or digital sensor of a camera. Once an image has been brought into focus thousand of receptor cells, called rods and cones, react to the light that strikes the retina."
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