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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 '14 at 22:23
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 '14 at 2:51
More to the point, if you got hot enough to glow red, you would be dead. Red hot starts at around 420C. – Simon B May 15 at 12:44

6 Answers 6

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 for more details.

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Not only the human body. Everything emits radiation. But the wavelength of this radiation depends on temperature.

The wavelength actually comes from the frequency of atom vibrations. If atom is hot it moves faster and generates higher frequencies of electromagnetic waves (shorter wavelength). Imagine shaking your hand in water where you would see that wavelength and frequency differ depending on the speed of your hand.

All atoms in your body have the temperature of your body and emit wavelength in your body frequency. All atoms outside your body also emit wavelength of its temperature.


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FYI : If you vibrate yourself fast enough you could glow in any wavelength. But before that you may got friction heat and burn yourself so don't try it at home – Thaina Jan 13 at 16:17
Awesome! that sounds like the screaming at a cup of coffee for decades to make it boil kind of activity - probably could do both at once! (good answer though +1) – user77400 Apr 15 at 8:47

Actually the human body emits more than thermal radiation. The Czech military did a study on measuring the extreme low frequency radio band emitted by the nervous system. It can be found on by searching

Human electromagnetic emission in the ELF band - Measurement ... › Lipkova

This makes perfect sense when you consider the electrodynamics of the nervous system. While we may have blood vessels and ion channels in the neurons themselves, as opposed to magnets and wire in a standard LC circuit, in essence it is a more discrete structure, with blood carrying hemoglobin, oxygen, and chemical energy, it is still essentially an inductor. The ferromagnetic interaction of the blood and the significantly higher charged ion transfer interneuron, would maintain an iduction of charge across the electrically active regions of the body.

Also, when considering that the nervous system is a high potential compared to the probable local environment, flesh itself may fulfill the role of a dielectric, thus allowing for a fundamental electromagnetic resonance to build up, generating dipole radiation.

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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

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

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This seems to point out information already known by the OP. – Kyle Kanos Feb 14 '14 at 21:58
It didn't before the question was edited. – m3z Feb 15 '14 at 9:08
A bit of a unnecessary negative tone in this answer.. – user77400 Apr 15 at 8:48

Below is my opinion,

  1. Actually, human body not only emit infrared radiation, it also all kind of different frequency waves or spectrum, but infrared radiation emitted from human body is higher intensity compare to other wavelengths. Our universe is containing few kind of current known fields such as electromagnetic field, gravity field, quantum field etc, scientist wish to unify these fields and the other unknown field to one may be called "Universal Field" in future. Every fraction of second, particles inside our body are moving rapidly, their movement are constantly disturbing the "Universal Field" in the space. Some particles in our body are vibrating extremly fast and most are not, therefore, we merely observed low frequency (large wavelength) radiation which is called infrared radiation.

  2. Once the "universal field" is being disturbed by an energy, thus, the energy start to transfer from one point to another point in space (we may call this as a photon). A photon is also known as one excitation or one disturbance of electric field (magnetic field) which carry some amount of energy. Heat is the excess energy released during the interaction of fields with matter and another matter. Thus, photon and heat is just describe the different state of energy, but they are the same quantity which is called ENERGY. The important context in the universe is the interaction between FIELD and ENERGY.

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