Why are Black and White not listed as Colours in Physics?

In physics, a colour is visible light with a specific wavelength. Black and white are not colours 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.

Why are Black and White not listed as Colours in Physics

Definition of Colour

The correspondence of colour to a specific wavelength is called spectral colour. White and black are excluded from this definition because they do not have specific wavelengths. White is not defined as a colour because it is the sum of all possible colours. Black is not defined as a colour because it is the absence of light, and therefore the colour. In the visual art world, white and black may sometimes be defined as distinct colours. This is different from the concept of spectral colour in physics.

How Do We Experience Color?

When we see colour, we experience the 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 colours 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 behaviour approaches that of a blackbody. This is why the light from the sun includes all colours of visible light; it is so hot that it behaves like a blackbody.  

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Compiled using excerpts from an article written by Serm Murmson.

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