# Light emitted by an object according to its temperature

According to this picture

the light emitted by an object depends on its temperature.

That makes perfect sense when we heat a metal. As its temperature raises we see it red at first, then orange, then yellow... and then?

If we heated it even more should we see the metal turning green? and then blue?

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You would be unlikely to see green. The problem is that in order to see green, you would need the spectrum of emitted light to peak at green and have relatively little contribution from other frequencies. This, for example, is the reason you do not see green stars (There is a cute Feynman story about this). An explanation of color temperature is given on wikipedia. Here is the path a black body takes with temperature increase--a Planckian locus diagram .

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Ok, so that diagram I linked is an oversimplification. The ratiation emitted is not just one wave-length, it's a range of wave-lengths. And if after yellow comes white I guess it means the object radiates the whole rainbow at that temperature. – GetFree Feb 5 '11 at 22:26
Are any factors at play here other than the temperature's peak frequency and the eye's RGB sensitivity? – Griffin May 22 '13 at 4:51

The phenomenon you're referring to is black body radiation. When a material gets hot, its electrons are in excited states. Occasionally, an electron will drop down to a lower state, emitting a photon. The higher the temperature, the higher the frequency of emitted light.

This is why the heating element in your oven turns red. It's also how incandescent light bulbs work. Most of the light we see from stars is due to black body radiation, and we can infer the temperature of a star by its color because of this.

The equation that gives energy emitted as a function of frequency and temperature is Plank's Law.

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So, will it turn green at higher temperature? – gigacyan Feb 5 '11 at 22:01
No. this answer describes the physics of the situation, but fails to explicitly explain why thermal emission never looks green. – Colin K Feb 5 '11 at 22:19

To see the green color, you have to use a green glass (a filter). The higher temperature, the brighter the green light emission.

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I'll show you an interactive diagram which lets you vary the temperature of a blackbody and see its effect on the radiated wavelengths.

But before you go there, recall that the visible spectrum is within the range of about $0.4$ - $0.8$ $\mu m$.

Here's the page with the diagram. As you increase the temperature, you'll find that the emitted radiation peaks over the range of wavelengths of visible light. So you're correct that the blackbody emits virtually the entire rainbow at very high temperatures.

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Your link gives peak wavelength. You have not drawn the line (correctly) between peak wavelength and color. A spectrum could hypothetically peak in the blue and appear almost completely red. – Alan Rominger Apr 7 '12 at 22:46