This page is about the human eye
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.
The two main parts of the eye that help in focusing light onto the retina are the cornea and the lens (most of this focusing is done by the cornea, and the lens makes the final adjustments). When light hits the cornea at an angle, the light is refracted. The cornea acts as a converging lens, and the light is bent inwards. Because the shape of the cornea is fixed, similar light rays are bent by the same amount. After this, the light rays pass through the lens, when there are refracted by a little more - enough to make them in perfect focus on the retina.
The lens, unlike the cornea, can refract the light rays by different amounts. As the lens is stretchy, it can be pulled into a thin shape, or it can be left in its normal fat shape. If the object you look at is close, and the light rays are spreading out when they reach the eye, they need to be bent. Therefore, the brain automatically 'tells' the ciliary muscles to contract: the ring becomes smaller, meaning the suspensory ligaments go loose, and the lens falls into into its natural fat shape. The makes the light rays refract very sharply - and they are therefore in focus on the retina. The changes in the eye when focusing on objects at different distances is called accommodation.
Rods and conesEdit
Rods and cones are the two kinds of receptors in the retina. They are different in which type of light they are sensitive to; the rods are sensitive to dim light, and the cones to bright light only. Only rods send messages to the brain at night. However, rods cannot sense different colours - when only rods are working, the image we see is in black and white. The cones are all packed together in one small part of the retina: the yellow spot. The rods are spread out all over the retina.