The Eye and the Sense of Sight
We perceive shapes, distance, movement, color, heat, and depth with our sense of sight. The organ for the sense of sight is the eye. The size of the eye about the shape of a ping-pong ball. The eye alone cannot make sight possible. It works with the brain, and on the outside, needs light to be present.
First, let us see what the eye is made of and how it works.
The eye consists of three layers: The outer layer consists of the sclera and cornea. The middle layer consists of the choroid, ciliary body, and iris. The inner layer consists of the retina.
When light falls onto an object (for instance, a bee coming towards you), the light is reflected and directed to our eyes. The light travels through the pupil and passes through the lens. The lens sharpens the image (of the bee), inverts it (turns it upside down), and displays it on the retina. It is inverted because the rays coming through the eyes are refracted and cross each other.
The retina is a very complex tissue made of optic nerves (photoreceptors) that are special nerves for detecting light. The optic nerves are the neurons that take the inverted image (impulse) from the retina and send it to a special part of the brain. The brain interprets it and tells you what the object is and what to do. All this happens within millions of a second.
Rods and Cones
The retina has two types of nerve cells: The rods and the cones. Scientists believe that each eye has about 120 million rods and 7 million cones. Rods are super-sensitive in the dark and help us see things in the dark. Cones are active in the light and help us see color. Special cone cells are sensitive to three colors: red, blue, and yellow. With these three colors, the cone nerve cells can render them into millions of beautiful colors. They are the nerves that work hard all through the day!
Sometimes the various parts of the eye do not work very well, and that can cause defects in vision. Sometimes the inverted image is focused in front or behind the retina, causing defects like farsightedness and nearsightedness. When the eyes see farther objects but have difficulty seeing nearer objects, we call it farsightedness (also called hyperopia or hypermetropia). The opposite is called nearsightedness (also called myopia). These defects can be corrected by wearing prescribed lenses.