The physical energy received by our eyes is called light. Objects present around us reflect light energy. Light energy varies in intensity. Physicists have studied light energy in great detail. According to them, vision starts with the electromagnetic radiation that objects emit. This electromagnetic energy is the light described in terms of wavelengths. A light could be visible, if the electromagnetic spectrum stimulates the eye and produces visual sensation.
The wavelength of visible light determines its color. It has been observed by physicists that the wavelength of red is longer than the wavelength for orange; the wavelength for orange is longer than that of yellow, and so on. In the seven colors of a rainbow, violet is having the shortest wavelength in comparison to other colors.
The eye is often compared with a camera. Both the eye and the camera receive visual stimulations. The process of receiving visual stimulations is the same in both the cases except the fact that an operator operates the camera because it is a machine, whereas the eye has its automatic mechanism of operation. Moreover, the structure and functions of camera are simpler in comparison to the eye. In the eye, the light energy is converted to a neural code understandable to our nervous system.
The human eye is roughly spherical in shape. It consists of three layers: the outer layer (sclerotic coat), the middle layer (choroid coat), and the innermost layer (retina).
Sclerotic coat (Outer layer):
The outermost covering of the eye is called the sclerotic coat or the sclera. It gives protection to the inner structure of the eye. It is white in color and hard in texture. The white portion of the eye that we can see from outside is the sclera. Six external muscles are attached to it. These muscles help the eye for upward, downward, and side movements.
The cornea is a part of the sclerotic coat. It is white, hard and transparent because it does not contain any blood vessels. As it is situated in front of the eye, it can be seen and touched from outside. It is curved and convez in shape. It is the part through which light rays enter the eye.
The space between the cornea and the pupil is called the anterior chamber, and is filled with a transparent substance called aqueous humor.
Choroid coat (Middle layer):
It is the second layer of the eye lying just interior to the sclerotic coat. This coat is filled with black-pigmented tissues to absorb stray light. It consists of blood vessels to supply nutrition to the retina. This layer ends at the back of the ciliary body to form the iris.
Located behind the cornea, and in front of the eye lens, Iris is a circular muscle tissue of dark brown color. The iris is opaque. It controls the amount of light that enters the eye, by either constricting the pupil, or expanding the pupil.
For example, in low illumination, the eye requires more light, for which the iris expands the pupil. Pupil becomes larger in size and allows more light. On the other hand, in dim illumination, the iris contracts the pupil so that less light can enter the eye. The iris surrounds the pupil.
Just behind the cornea, pupil is the aperture other opening between the two irises. It is through the pupil, light rays enter the eye. The size of the pupil is controlled by the constriction and relaxation of the iris.
In the back of the pupil and iris, there is a rubbery, bean-shaped, transparent and crystalline lens, which bends the light rays onto the retina. It is surrounded by a circular mass of smooth muscles called the ciliary muscles. The ciliary muscles control the focusing of the lens for near and far vision. The lens becomes convex and thickening when we are looking at near objects and becomes concave and thinning when we are looking at distant objects. This adjustment process of the lens is called accommodation.
The space between the lens and the retina is called the oosterior chamber and is filled with a transparent substance called vitreous humor.
Retina (Innermost layer):
This is the innermost layer of the eye. It is compared with the photo film of the camera. The basic function of the eye is to convert the light waves into neural signals that the brain can process. This function is carried out in the retina.
The retina has several layers consisting of five types of neurons:
(a) the receptor sells called the rods and cones,
(b) the bipolar neurons,
(c) the ganglion cells,
(d) the horizontal cells, and
(e) the amacrine cells.
Retina contains the real visual receptor cells of the eye i.e., the rods and the cones. The rods and cones appear at the rearmost layer of the retina. There are about 125 million rods, and 7 million cones. The rods are long and cylindrical, and the cones are short, thick, and tapered. The rods and cones are distributed unevenly throughout the retina. The cones are concentrated more at the center of the retina, while the rods are spread more in the peripheral regions of the retina. In the center of the retina is a very small region called fovea that contains nothing but densely packed cones. The fovea is rod-free, and is the area of our sharpest vision. Both color and spatial details are most accurately detected in the fovea. If you would focus on something of particular interest, you will probably center the image from the lens onto the fovea.
The rods and cones are not only structurally dissimilar, but they also play clearly different roles in vision. The rods are responsible for black and white vision, while the cones are helpful for color vision. The cones are active in bright illumination, whereas the rods are active in dim illumination. The rods play a key role in peripheral vision, i.e., seeing objects that are outside the main center of focus, while the cones record sharp visual details. Both are necessary to interact with the nature, both in bright light as well as in dim illumination.
Besides, rods and cones, the retina contains other types of cells, as mentioned earlier. The bipolar cells are neurons, which combine impulses from the rod and cones and send the results to the ganglion cells. The ganglion cells combine the impulses from many bipolar cells and carry the visual information from the eye to the brain. The axons of the ganglion cells form the optic nerve. It is the optic nerve, which carries message to the brain. As the optic nerve exits the eye, it leaves a small area of the retina without the receptor cells. This area is called the blind spot. The blind spot contains no rods and cone, and therefore, vision is not possible in this area. As it is not sensitive to any light, it is called the blind spot. We do not normally experience blindness because what one eye misses, the other eye registers, and the brain fills in the information that is most likely missing.