Why Do We Not List Black and White as Colors in Physics?
Visible light, radio waves, x-rays and other types of radiation are all part of the electromagnetic spectrum. Visible light is electromagnetic radiation with wavelengths ranging from roughly 400 and 700 nanometers. In physics, a color is visible light with a specific wavelength. Black and white are not colors because they do not have specific wavelengths. Instead, white light contains all wavelengths of visible light. Black, on the other hand, is the absence of visible light.
Definitions of Black and White
The correspondence of a color to a specific wavelength is called spectral color. White and black are excluded from this definition because they do not have specific wavelengths. White is not defined as a color because it is the sum of all possible colors. Black is not defined as a color because it is the absence of light, and therefore color. In the visual art world, white and black may sometimes be defined as distinct colors. This is different from the concept of spectral color in physics.
How Do We Experience Color?
When we see color, we experience light of that wavelength reflecting off an object or emitting from it. For example, when light from the sun shines on your blue t-shirt, the t-shirt absorbs all of the light other than the blue light. The t-shirt reflects the blue light and some of it reaches your eyes. Black is the absorption of all visible light. This is also why your black t-shirt is hot on a sunny day. All of the colors of visible light are absorbed into a black object, heating it up
Where Does White Light Come From?
When objects generate extreme amounts of heat, they produce radiation across the entire electromagnetic spectrum, including visible light. This process is called blackbody radiation. A blackbody is a theoretical object that absorbs all radiation. Consequently, it heats up and then emits radiation across all wavelengths. As real world objects become hotter, their behavior approaches that of a blackbody. This is why the light from the sun includes all colors of visible light; it is so hot that it behaves like a blackbody.
How to See for Yourself
You can use a prism to visualize why white is not a color. If you shine white light on a prism, the light is refracted into all different colors. Refraction is the process of light bending as it passes through a medium. Different wavelengths of light are refracted at different angles by the prism, hence the visual rainbow. White light contains all the various wavelengths that you see coming out of the prism.