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So I was reading about EM waves and radiation. My textbook says that frequency of light depends on the temperature of a body. As the frequency rise up, the color shifts from red to blue. For example, copper. This is supposedly true.

I have this weird doubt - if copper changes its color with temperature, how do crayons have different colors? Crayons are made of same material and have a constant temperature. In fact how do any of the objects emit different colors, all being at same temperature.

I do not know if I am missing a basic crux about this argument. I would really like to know. Thanks in advance! :)

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You are missing the difference between emitted and reflected light. They all look different because they reflect different wavelengths, but if you look at them with a thermal camera you will see that they emit the same infrared color. It will be impossible to distinguish their hue.

To stimulate your curiosity, emission is not always dictated by tempertature. Lasers, LEDs and "neon" lamps (which usually contain mercury) emit light differently from one another and from thermal radiation.

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The light you see from the crayons is reflected by them. If the color is say yellow, that means that the crayon mostly absorbed and scattered all other wavelengths, and reflected the yellow one

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  • $\begingroup$ A nice way to see this is to think about looking at the Earth from space. what colour is is? Well, it's blue-green, right? Well, what colour is it on the night side? It's black, but in fact it's some 'colour' in the infra-red which we can't see. The first colour, blue-green, is what the Earth looks like in reflected sunlight, while the second, in the infrared, is what it looks like in the light it emits. And this second colour would indeed get bluer if the Earth was heated up a lot (the first colour would also change, probably to the white of clouds initially, as the oceans boiled). $\endgroup$
    – user107153
    Nov 16, 2016 at 11:46
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Colour temperature only makes sense for black body radiators, and even then, the colour doesn't simply "shifts from red to blue", but follows a specific curve known as the Planckian locus:

enter image description here

As you can see, most colours visible to the human eye (and available in crayons) are simply not reachable to black-body radiation, no matter the temperature. There are no green or magenta stars out there. So speaking about temperature of coloured light (both emitted and reflected) simply makes no sense.

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