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What causes humans to emit infra red radiation and why don't we also emit other types of electromagnetic radiation such as ultra violet or microwave? Sunlight contains UV and our body takes it in, but when we give off radiation, do we not radiate it back?

I know that the constant motion of molecules embodies heat, and that when an electron is excited by colliding with another, it jumps to a higher energy level and falls back again, releasing energy in the form of photons. My question is, then, does this energy 'in the form of photons' carry heat? Or is heat another form of energy release that is separate from EM radiation?

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The UV radiation is absorbed by the body and through complicated non-linear processes is converted to heat. So no reflection of UV radiation takes place. –  Urgje Feb 14 at 22:23
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I kinda get it. Human can't be that hot because we have the mechanism to maintain a stable body temperature, we are endotherm. –  user40003 Feb 15 at 2:51

3 Answers 3

Kyothe was on the right track, but in fact we do radiate in the visible, just in such small amounts that it's not detectable for all practical purposes. If you look at the referenced Planck (black body) curves for objects around human body temperature, the short-wave tail is nonzero in the visible range, but it's there.

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We radiate infrared rather than UV or visible light because we aren't hot enough. See http://en.wikipedia.org/wiki/Planck%27s_law for more details.

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A little google searching would have had you your answers pretty quickly. But I was bored so here they are:

In answer to the first question:

"The wavelength of radiation emitted depends on the temperature of the objects. Such radiation is sometimes called thermal radiation. Most of the radiation emitted by human body is in the infrared region, mainly at the wavelength of 12 micron." quoted from http://www.hko.gov.hk/education/edu02rga/radiation/radiation_02-e.htm

In answer to the second question:

"Infrared Radiation is electromagnetic radiation with wavelengths between those of visible red light and radio waves. Infrared waves are given off by all warm objects... The waves cause heat by exciting (increasing the movement of) molecules in the substances they strike." quoted from http://science.howstuffworks.com/infraredradiation-info.htm

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This seems to point out information already known by the OP. –  Kyle Kanos Feb 14 at 21:58
    
It didn't before the question was edited. –  m3z Feb 15 at 9:08

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