When light contains waves of one wave length (to be precise within a very narrow range of wave length) we feel a color from such light. We call it a pure spectral color. For example we feel yellow from light whose wave length is about 580 nanometers. When we feel a yellow color under the sunlight there must exist a light of about 580 nanometers wave length.
What is interesting here is that even if we feel a yellow color it does not necessarily mean there exists a light of 580 nanometer wave length. When we see a yellow color on a computer display it does not come from a light of 580 nanometer wave length. All Colors on a computer display are made of three primary colors, i.e. red, green, and blue. When we feel a yellow color it is actually a combination of lights of red and green.
We can not distinguish a light of 580 nanometers from a combination of lights of red and green. It is a kind of illusion in our brain. We strongly feel that there exists a yellow color on a computer display and there exist a pure yellow light but it is actually a combination of lights of red and green.
More interestingly, a rainbow does not include a magenta color. This means there is no pure light corresponding to a magenta color. We feel a magenta color only from a combination of lights of red and blue. We feel there is a magenta color and therefore there is a magenta light but in reality there are red and blue lights.
What you see does not always equal what exists.
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As you studied in a physics class light is wave and its wave length determines its color. Light with a wave length of about 700 nanometers is red while one with 400 nanometers is violet. Usually light contains many waves of various wave lengths. For example a ray of sunlight contains waves of all wav