Science Questions with Surprising Answers
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Dr. Christopher S. Baird

Why are there only six fundamental colors: red, orange, yellow, green, blue, and violet?

Category: Physics      Published: December 4, 2012

the visible spectrum
A prism splits white light into the fundamental spectral colors. Because the spectrum varies smoothly, there are an infinite number of fundamental colors. Public Domain Image, source: Christopher S. Baird.

There are an infinite number of fundamental colors, if by "fundamental" you mean "spectral". Spectral colors are also known loosely as rainbow colors. A spectral color is composed of a single fundamental color on the visible part of the electromagnetic spectrum, as opposed to a mixture of colors. Spectral colors such as red or green are composed of light waves of a single frequency. Non-spectral colors such as brown and pink are composed of a mixture of spectral colors. Simple lasers, by their very nature, only emit one frequency of light (to an excellent approximation). This means that simple lasers can only generate spectral colors (compound colors from laser systems can be made by mixing the light from several lasers, or by running the spectral color through a material that converts it to a mixture of colors). Even though the spectral colors are a subset of all colors, there are still an infinite amount of spectral colors. This fact becomes obvious when looking at the spectrum from a prism, which is simply a spread of spectral colors.

Note that an atmospheric rainbow is not a spread of pure spectral colors. Although it is close to a pure spectrum, a rainbow in the sky really consists of mixed colors. For this reason, it is incorrect to refer to a pure spread of spectral colors as a "rainbow" even though they look similar. A pure spread of spectral colors can be produced by passing perfectly white light through a prism or a diffraction grating. A pure spectrum does not have six solid bands of color. Rather, a pure spectrum has a smooth variation of colors. Red and orange are spectral colors, but so is the color half-way between red and orange. You don't have to make this color by mixing red and orange. It exists on its own as a fundamental color with its own frequency. The same is true of all the in-between spectral colors that don't have common names.

Because the physical color spectrum is continuous, the naming of colors is purely a societal affair. For instance, Americans identify the color "yellow" as the color next to green with no orange tint. But Germans use the word "gelb", which translates to "yellow", to identify colors that span from yellow to yellow-orange. If a yellow-orange truck drove by, the American would call it orange and the German would call it yellow. Who is right? They are both right because they are just using names differently.

It is a good thing that fundamental frequencies (colors) come in a continuous spectrum and not just in a few discrete options. Otherwise, we would not be able to enjoy so many radio stations. Each radio station broadcasts radio waves at a different frequency to avoid interference with each other. If there were only six "colors" to the radio part of the spectrum, we would be stuck with six radio stations. This principle also allows many cell phones in the same room to communicate with a cell phone tower without interfering with each other.

Topics: color, color mixing, color theory, colors, light, spectral colors, spectrum