Within the electromagnetic spectrum, different wavelengths of visisble light appear to us as different colors. Light that contains all wavelengths in the same proportions, such as sunlight appears white.
When a beam of sunlight passes through a triangular transparent object known as a prism, the rays of different wavelengths are refracted at different angles. The bending breaks up the sunlight into a band of colors. This band contains the visible spectrum. At one end of the spectrum, the light appears as violet. It consists of the shortest wavelengths of light that we can see. Farther along the spectrum, the lights wavelengths get increasingly longer. The spectrum appears as blue, green, yellow, orange, and red, each shading into its neighboring colors in the spectrum. A deep color of red uses the longest wavelengths of light that we can see.
Light waves are a form of electromagnetic radiation, which consist of patterns of electric and magnetic energy. The visible spectrum is only a small part of the electromagnetic spectrum--the entire range of electromagnetic waves. Beyond the violet end of the visible spectrum are ultraviolet rays, X rays, and gamma rays. Beyond the red end of the visible spectrum are infrared rays and radio waves.
Such everyday objects like traffic lights and neon signs appear colored. This is because the light that they give off contains a limited range of wavelengths. However, most objects appear colored because their chemical structure absorbs certain wavelengths of light and reflects others. For example, when sunlight strikes a carrot, molecules in the carrot absorb most of the light of short wavelengths. Most of the light of longer wavelengths is reflected. When these longer wavelengths of light reach our eyes, the carrot appears orange.
An object that reflects most of the light of all wavelengths in nearly equal amounts appears white. An object that absorbs most of the light of all wavelengths in nearly equal amounts appears black.
How we see color
The roles of the eyes and brain.
The ability to see color depends on the many complicated workings of the eyes and brain. When we look at an object, light coming from the object enters our eyes. Each eye focuses the light, which forms an image of the object on the retina. The retina is a thin layer of tissue covering the back and sides of the inside of the eyeball. It contains millions of light-sensitive cells. These cells absorb most of the light that falls on the retina and convert the light to electrical signals. These electrical signals then travel through nerves which lead to the brain.
The retina has two main types of light-sensitive cells--rods and cones. These two cells are named after their shapes. Rods are extremely sensitive to dim light but cannot distinguish wavel engths. For this reason, we see only tones of gray in a dimly lit room. As the light becomes brighter, the cones begin to respond and the rods refrain from functioning. The retina of a person with normal color vision has three types of cones. One type responds most strongly to light of short wavelengths, which corresponds to the color blue. Another type reacts chiefly to light of middle wavelengths, or green. The third type is most sensitive to light of long wavelengths, or red.
The brain organizes nerve signals from the eye and interprets these signals as colored visual images. Exactly how the brain makes us aware of colors is still very much of a mystery. Scientists have developed several theories to explain color vision.
Some people do not have full color vision. Such people are said to be color blind. There are different types and degrees of color blindness, depending on different abnormalities in the retina's cones. In severe cases, one type of cone may be absent or not functioning. People who have such an abnormality confuse certain colors with others. Very few people cannot see colors at all. Most color-vision problems are inherited and cannot be cured.
Surprising color-vision effects.
Many operations of the eyes and brain work automatically and almost instantly in providing us with color vision. We have unconsciously learned not to "see" certain visual effects of these operations, especially while our eyes adjust to changes of color. When we do become aware of these effects, they may seem dramatic or startling. Some of the color-vision effects that we normally do not notice can be easily demonstrated.
One color-vision can be demonstrated by covering half a sheet of brightly colored paper with plain white paper. If we stare at the colored area for about 30 seconds and then remove the white paper, the area that had not been covered will seem much lighter than the half that had been covered by the white paper. It seems lighter because our eyes adapt to (become accustomed to) colors. Such a visual effect is called chromatic adaptation.
If we stare at a colored image for about 30 seconds and then look at a white surface, we see an afterimage. The afterimage is the same shape as the original image but different colors. If the original image was red, the afterimage will be green. Where the image was green, the afterimage will be red. Blue areas become yellow, and yellow areas become blue. Black and white also reverse. The technical name for this amazing color-vision effect is successive contrast.
We can also demonstrate that the appearance of a color is influenced by surrounding colors. If we place the same color against different background colors, the color will look different in each case. Also, a color appears lighter when surrounded by a dark background than it does when surrounded by a light background. This color-vision effect is called chromatic induction or simultaneous contrast.
Sometimes, we may see colors in areas that are only black and white. These such colors are called phantom colors. Phantom colors may be seen by staring at flashing black-and-white patterns, such as those produced by a rapidly rolling black-and-white television picture.