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The Real Colors of the Rainbow

 , September 09, 2018
Rainbow colors are colors that are present in sunlight. These are traditionally listed as red, orange, yellow, green, blue and violet. These colors are arbitrary as humans can see hundreds of different colors in a rainbow but people tend to merge these into color bands.

Color Bands in a Rainbow

A rainbow is caused by the reflection, refraction and dispersion of light by droplets of water in the sky. Specifically, they occur when light is refracted when entering droplets, reflected on the back of the inside of droplets and then refracted again when exiting the droplet. This breaks white light into its component parts. The eye can see the following wavelengths of light:
Violet380–450 nanometers
Blue450–495 nanometers
Green495–570 nanometers
Yellow570–590 nanometers
Orange570–590 nanometers
Red590–620 nanometers
These are spectral colors that result from a single wavelength of light. Although people group the colors into 5 or 6 bands, it is theoretically possible to see hundreds of spectral colors in a rainbow. For example, it is common to see turquoise between blue and green.

Double Rainbow

A double rainbow occurs when light is reflected twice inside each water droplet. This tends to be visible in perfect conditions with strong sunlight low in the horizon with dark colors overhead. The colors in the second arc of a double rainbow are in reverse order.

Colors of the Rainbow

A spectral color is a pure wavelength of light that is perceived as a color. Visible light is in a continuous spectrum from 380 to 620 nanometers. Wavelengths close enough together are indistinguishable by the eye. The range 380-620 is arbitrarily divided into five or six colors. This is usually listed as red, orange, yellow, green, blue, indigo and violet. Some people merge blue and indigo. In reality, the eye can distinguish hundreds of colors based on different pure wavelengths of light. The spectral colors correspond to the colors of the rainbow.

Colors Not Found in a Rainbow

Rainbows produce spectral colors that are pure wavelengths of light. Beyond the hundreds of spectral colors, the brain perceives about 10 million colors based on different mixes of wavelengths. For example, colors such as white, black, brown and pink are non-spectral colors. Black is a special case that can be perceived as no light at all. In practice, colors perceived as black do contain mixed wavelengths of low intensity light.

Color Myths

It is common to confuse the terms spectral color and color. For example, it is occasionally argued that white, black, pink or brown aren't colors because they result from mixed wavelengths of light. Color is not a classification system for pure wavelengths of light, it is a type of human perception. As such, if the brain perceives a color, it is a color. Myths about color are typically based on simplified tools such as a color wheel that take hundreds of spectral colors and millions of non-spectral colors and place them into an easy to use system of 5 or 6 colors. For example, people may feel that any color not on a particular color wheel isn't a "real" color. Likewise, a rainbow serves as a natural reference for color and people may make incorrect assumptions based on rainbows such as guessing that their are only five real colors.
Overview: Rainbow Colors
DefinitionSpectral colors that can be viewed in the sky under conditions where sunlight strikes water droplets in the air causing the droplets to divide white light into its component wavelengths.
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Kaiser, Peter K., and Robert M. Boynton. "Human color vision." (1996).
Boynton, Robert M. Human color vision. Holt McDougal, 1979.

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