The eye is a sense organ, and contains receptors which sense light. These receptors are of two types: rods and cones. These are found in the retina, in a thin layer at the back of the eye. When light falls onto these rods and cones, the light energy creates tiny electrical signals in them. These then go to the brain, along the optic nerve - taking less than a second to reach there. The brain will then interpret these signals so that we can 'see' an image. The clarity of this image depends on how clear the image made by the light falling on the retina is. Therefore, the parts of the eye near the front focus the light, which then forms a clear image on the retina.