Refer to the eye diagram. Starting from the outside, note the following parts. First, start with the cornea, or the clear outer covering of the eye. Contact lenses sit on the cornea. Behind the cornea is the iris. The iris controls the amount of light that enters the eye by forming the pupil. The pupil is the hole formed by the action of the cornea. Behind the pupil is the lens, the lens focuses the light so that it lands on the retina. The retina is where the photosensitive cells are located. In particular, the lens focuses light so that when you stare directly at an object, the light hits the fovea. The fovea is a special part of the retina.
The retina is composed of a number of layers. The top three layers are involved in vision. The thirdlayer down is where the photosensitive cells, the rods and cones, are located. So, light must travel through the two other layers of cells before it can be detected.
In the human eye, the cone cells are located primarily in and around the fovea. The cones are responsible for color vision and daylight vision. The cones also provide us with our sharpest vision, or highest acuity of vision. Finally, the cones are much less numerous than the rod cells.
The rod cells are responsible for night vision and for seeing in black and white. So, people who are totally colorblind must not have functioning cone cells. Further, at night, you cannot see colors (you just think that you do). But, that is a topic for the next chapter.
In the human eye, rods are found everywhere in the retina, except in and near the fovea. Rods do not detect light as sharply as the cones do, but rods are much more sensitive to low light levels than the cones are. Finally, there are many more rods than there are cones.
An interesting side topic is the problem of a detached retina . Detached retinas are often the result of trauma. Suppose the retina has been knocked off of the top of the eye and has folded down. Naturally, the photoreceptive cells will not work in this case because they are not facing the proper direction. In that case, the person will see out of only a part of the affected eye. Today, it is possible in some cases to reattach the retina using microsurgery.