Ask a Physicist

Is there any truth to the idea that dark colors attract heat and light colors do not?
Submitted by Mary Lyman from New Windsor, New York

You could say something like that, though "attract" isn't really the right word. If you leave a dark car and a light car out in sunlight, you'll notice after a while that the dark one feels hotter. But it's not that the dark car is actually attracting any more heat. They're both getting the same amount of light and heat from the Sun, but the dark one absorbs most of it while the light one reflects most of it. That's why the light one looks light - the photons are bounced to your eyes instead of disappearing into the material and heating it up.

It's different if you talk about things that are emitting light (and heat) of their own. You often hear physicists and astronomers talking about the Sun itself as a "blackbody," which might sound very strange since you can get blinded by looking at it. But that refers to the fact that the Sun reflects very little compared to how much it emits. And it turns out a good emitter is also a good absorber, so in the sense of absorbing light rather than reflecting it the Sun could actually be called very dark.

It turns out everything emits light, or electromagnetic radiation, at various wavelengths. The hotter it is, the more it emits, and the more of what it emits is at shorter wavelengths. Even cold space is full of microwaves, warmer things like humans emit mostly infrared (which we feel as heat), medium-hot stars like the Sun emit mostly visible light, very hot neutron stars emit x-rays. If you leave that dark car out in the sunlight long enough, it will heat up enough that you can feel the warmth without quite touching it. That's from its emission. It'll never get hot enough to emit visible light, like what it absorbed from the Sun - it has to lose a little energy in the turnaround, which means longer wavelength. But when it's hot it emits infrared, which is the next longer wavelength after visible light.

So you could say that dark materials are better at absorbing light, which heats them up - and they're also better at sending the heat back out.