Light is a funny thing.
It behaves like an electromagnetic wave (it has a phase - it can undergo interference), yet it also behaves like a stream of particles (it consists of photons, as proven by Einstein's photoelectric effect). The split personality exhibited by light has bewildered scientists for one hundred years. And, while we will never see a photon with our naked eye, our eyes would see nothing at all if not for light. These photons reveal the nature of the matter around us, but keep their own true nature under wraps.
The light that reaches our pupils may come directly from a source or it may arrive after a series of reflections off of other surfaces; either way, the light emanates from a source, like the sun or a light bulb. Both of these particular sources produce what is known as white light.
All electromagnetic waves, whether they are radio waves or x-rays, have a wavelength associated with them. Visible light fills a very narrow band within the electromagnetic spectrum: 400 nm to 700 nm. All wavelengths in this range correspond to a particular colour. Violet light has a wavelength of 400 nm, while that of red is 700 nm. A rainbow will have these colours on either side (violet on the bottom, red on top), and all other colours in between ordered by wavelength. White light is what you get if you superimpose all colours in the visible spectrum on top of one another. Black, on the other hand, represents the absence of colour.
With this background information on light, we can properly appreciate why a red sports car looks like a red sports car. It is less obvious than you might think...
The sun undergoes fusion, a nuclear reaction, which produces, among other things, white light. These white light rays travel in all directions at about 300,000 km/s. After travelling a little over eight minutes, some of these white light rays hit the red sports car that sits before you. Then, something critical happens: some of the white light is reflected by the car's surface, but most of it is absorbed. In fact, the surface absorbs every wavelength of light with the exception of its own, red, 700 nm. The 700 nm wavelength of the light is reflected by the surface. These reflected light rays head off in many directions, including yours. Your brain interprets this wavelength to be red.
So, while the car may be red, we could not make this deduction if not for a source of light that contains, at a minimum, this particular wavelength. It is kind of like that falling tree... If an object is not lit up by a source, does it have a colour?
The car's red colour gives it a certain character. And, while one's personality is more complex than a single data point along a spectrum, colour codes for personality classification are often surprisingly telling.
