How is color affected by wavelength?
What we see as light is part of an extensive range of "electromagnetic radiation". Each band of electromagnetic radiation blends into the next but has its unique characteristics. Some, like radio, can be carried over great distances. Some, like X-rays, penetrate thick steel or destroy human tissue. Most of this radiation, however, is not directly visible to the human eye. Our eyes are only sensitive to a narrow range from approximately 400 nm to 700 nm. This restricted wavelength range is therefore called the visible spectrum.
When a relatively uniform mixture of all visible wavelengths is produced by a light source, the lighting appears white and colorless. But if only a few wavelengths are present, the light appears colored. For instance, wavelengths from approximately 400 nm to 450 nm are considered dark purple.
It will turn blue if wavelengths are changed for 450–500 nm. From 500 nm to 580 nm the light appears to be more blue-green, and from about 580 nm to 600 nm we see yellow. The yellow becomes more orange if the light wavelengths get longer; at 650 nm, it becomes red, becoming darker when the response limit is reached at 700 nm. This means that the colors of the spectrum – purple, blue, green, yellow, and red – are all present in different types of white light.
The human eye appears to contain three types of light receptors, corresponding to wide bands of overlapping blue, green, and red wavelengths. When all three receptors are equally stimulated by something you see, you tend to feel it as white, or neutral gray.
The stimulation is uneven if there is a large wavelength imbalance – maybe the light has a lot of redder (long) waves than blue (short) waves.
The light in this case can look tinged with orange, as occurs each day at sunrise or sunset.
Try to recall the order of the colors of the visible spectrum.
It is useful when you need to figure out the color response of black-and-white films or choose color filters and darkroom-safe lamps.