# Why does a human body only emit infra red radiation and not other types of electromagnetic radiation?

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?

• 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. Feb 14, 2014 at 22:23
• possible duplicate of All objects radiate energy, but we cannot see all objects in the dark. Why?
– Danu
Feb 15, 2014 at 1:01
• I kinda get it. Human can't be that hot because we have the mechanism to maintain a stable body temperature, we are endotherm. Feb 15, 2014 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. May 15, 2015 at 12:44

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.

• What about UV? solid above $0$k emits EMR in a continuous spectrum right? So the probability of human emitting high energy photon (in the UV range) is very, very low but still there? Nov 8, 2020 at 6:39

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.

• 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 Jan 13, 2015 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, 2015 at 8:47

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.

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 www.measurement.sk by searching

Human electromagnetic emission in the ELF band - Measurement ... www.measurement.sk › 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.

Humans also emit non-trivial amounts of ionizing radiation. The source of this is mostly radioactive potassium isotopes consumed via foods like bananas.

• Not just potassium and not just from banana. :-) hps.org/publicinformation/ate/faqs/faqradbods.html Jan 1, 2018 at 19:05
• Which is why I said mostly from potassium isotopes and from foods like bananas. The link you provided is a much longer version of what I just said. So why the downvote?
– Slam
Jan 2, 2018 at 20:33
• Others downvoted it, I upvoted... Jan 3, 2018 at 11:03