Why certain colours absorb heat and others reflect it?


How is it possible that certain colours absorb heat and others reflect it? They are just ‘colours’?
Why if you are wearing black pants, you get warm, but with white, less so – even if it’s the same material?

In: 3

For an object to be a certain colour, that means it is absorbing the other wavelengths of light but reflecting the wavelengths corresponding to that colour. So if an object is blue it is absorbing other wavelengths but reflecting the wavelengths that correspond to blue light. Since white light is made up of all the wavelength that means that of something is white it is reflecting lots of light, the opposite is true for black it is absorbing all the light. Since light is a great way of transferring energy this energy is absorbed by black clothing and turned into heat, another form of energy.

It’s not that some reflect and some absorb.

All materials are on an albedo scale, from 0 to 1.

At 0, an object will absorb all light, and reflect nothing. This would be completely black. It would also radiate heat very well; if warm and not being heated up by an external source, it would quickly radiate that heat.

At 1, exact opposite. It will reflect all light, and absorb nothing. It would look like a perfect mirror. It would not radiate heat. If hot, and not heated by an external source, it would remain hot.

In terms of clothes, this has implications. On a sunny day, not exorcising much, the primary source of heat is the sun, so wearing black makes you very hot. Wearing white, with a higher albedo, you reflect the sunlight more and stay cooler. But at night, if you are exorcising, it flips. Now the primary source of heat if your body, so wearing black will help you stay cool, and if you want to keep warm, you should wear something as reflective as possible; hence why emergency blankets look like tinfoil.

(This is also why radiators should be black, like the one in your car, but interior designers around the world all failed thermodynamics, so enjoy your higher heating bill and wasted energy)

Colour is just the wavelength of reflected light. A black object is reflecting very little, a blue object is reflecting light in the blue wavelength very well, but absorbing reds more, a white object reflects most colours pretty well, and a mirror reflects everything. In that sense, your question is backwards; colour is just reflecting radiation (heat).

The colour isn’t “just” a colour. It’s a colour because that is the light reflecting back at you.

Imagine a wall 2m x 2m and we’re gonna throw a bunch of different sized balls at it. If we put a 1m x 1m hole in it, most of the balls are going to go through with very few coming back to us for a second throw; this is black. If we split that 1m x 1m hole into a hundred 10cm x 10cm holes we have the same ratio of hole to wall [3m^2 of wall and 1m^2 of hole] but now the volleyballs, basketballs, and footballs bounce back to us while most of the golf balls, baseballs, and ping pong balls go through; this is a colour. If we split the hole up into ten thousand 1cm x 1cm holes, then we still have the same ratio of hole to wall, but now almost everything bounces back; this is white.

This is a physical example rather than an electrochemical example but the very basic principle is the same. The ‘material’ of the clothes has not changed but the ‘colour’ you see is just a certain arrangement of ‘holes’. Now go enjoy your basketball-coloured pants.

Colours are our perception of how objects are reflecting light. Light colours, especially white, mean the object is reflecting most of the visible light. Light that’s absorbed causes an object to heat up, so black clothes will generally feel warmer in the sun than white ones. The effect works both ways, so black objects will tend to cool faster in the shade, which is why radiators and heat sinks tend to be black.

Much of the sun’s energy reaches us as infrared light, which we can’t see and so it doesn’t affect colour. This means that you could have a white object that absorbs infrared and so heats up in the sun. You could also have a black object that reflects infrared that wouldn’t heat up as much. The rule that light-coloured things stay cooler in the sun is a reasonable starting point but it’s not always true because we can’t see infrared light.