The retina has a complex arrangement of light-sensitive receptors known as rods and cones because of their shape. The cones are color-sensitive and require higher levels of illumination. The rods are far less color-sensitive and come into play when the light is dim. That is why a dimly-lit scene will also have very little color information in it. Near the center is the fovea, about 0.3 mm in diameter where the density of cones is extremely high. In detailed work requiring a great deal of information, like reading, the image is placed on the fovea for maximum information and color discrimination. Figure 17.15 shows the retina, with its light sensitive rods and cones, as viewed under a microscope. Human vision is a complex process involving the retina of the eye and the cortex of the brain. Edwin Land (of Polaroid fame) coined the word "retinex" to indicate the integral part played by both the retina and the cortex.
Where the optic nerve attaches to the retina there are no rods or cones. This means that each eye has a small blind spot. Normally we are not even aware of this because when an image is not seen by one eye because of its blind spot an image will be clearly seen by the other eye. However, it is easy to demonstrate this blind spot with Figure 17.16. Shut your left eye, hold your book at arm's length, and look directly at the o but be aware of the x. Now slowly bring the book closer to you, still looking directly at the o. At some distance the x will disappear, its image will pass over the blind spot of your right eye. To demonstrate the blind spot in your left eye, close your right eye and look directly at the x but be aware of the o. Vary the distance of the book from your eye and you will notice the o disappears when its image passes over the blind spot on the retina of your left eye.
As you already know, the human eye is sensitive to light with wavelengths from about 700 nm, which we see as red, to about 400 nm, which we see as violet. There are three kinds of color-sensitive cones with maximum sensitivities each to light with wavelengths of 575 nm, 535 nm, and 445 nm. We can refer to these three kinds of cones as being sensitive to red, green, and blue light, respectively.
We see the color of an object by the light that it reflects. When white light, such as sunlight, shines on an object that means it is illuminated with light of all wavelengths. If red light is reflected while light of other colors is absorbed, we will see the object as red as shown in Figure 17.17. There we have used incoming rays of red, green, and blue to indicate the full range of colors that is in white light. Objects that we see having different colors reflect different colors from white light and absorb the rest; this is illustrated in Figure 17.18 for a green object and a blue object.
We have used red, green, and blue to represent the many colors present in white light such as sunlight and because these colors correspond to the three kinds of color-sensitive cones in our retinas. However, these color-sensitive cones are sensitive over a fairly wide range of wavelengths or colors. And very few objects absorb or reflect light over a very short range of wavelengths. Figure 17.19 illustrates what we see when white light shines on something that absorbs light from the red end of the spectrum while the rest of the light is reflected. White light without its reds and oranges will be bluish-green or aqua-colored or turquoise; we call this color cyan.
We see objects of all colors by the colors of the light they reflect. And the colors of the reflected light depend upon the colors present in the light that illuminates the object. Your car may appear quite different when you look at it in sunlight or on a parking lot illuminated by the warm red-orange glow of sodium vapor lights or the cool bluish-white of mercury vapor lights. Your own clothes may look quite different when you see them under the ordinary incandescent light of your room or the fluorescent lights of your workplace or outside in the sunlight. We are used to the white light of sunlight with its distribution of colors. Incandescent lights give off more red and orange and less blue and green and we may refer to such light as "warm". Fluorescent light has more blue light and less red and orange and we may refer this light as "cool".
A: To get all the information you need for letter and word recognition, the image of a particular word or letter must be focused on the fovea where there is a very great concentration of cones, the color-sensitive receptors on the retina.
A: While most of the focusing of light by the eye occurs at the cornea, the fine adjustment of the focusing of the image is made by the scillary muscles acting on the crystalline lens. As eyes (and people) age, the crystalline lens looses some of its ability to change its shape. Bifocals are a compensation for this loss of pliability.