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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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  • $\begingroup$ 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. $\endgroup$
    – Urgje
    Commented Feb 14, 2014 at 22:23
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    $\begingroup$ possible duplicate of All objects radiate energy, but we cannot see all objects in the dark. Why? $\endgroup$
    – Danu
    Commented Feb 15, 2014 at 1:01
  • $\begingroup$ I kinda get it. Human can't be that hot because we have the mechanism to maintain a stable body temperature, we are endotherm. $\endgroup$
    – user40003
    Commented Feb 15, 2014 at 2:51
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    $\begingroup$ More to the point, if you got hot enough to glow red, you would be dead. Red hot starts at around 420C. $\endgroup$
    – Simon B
    Commented May 15, 2015 at 12:44

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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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  • $\begingroup$ 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? $\endgroup$
    – CountDOOKU
    Commented Nov 8, 2020 at 6:39
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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.

FYI http://en.wikipedia.org/wiki/Black-body_radiation

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  • $\begingroup$ 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 $\endgroup$
    – Thaina
    Commented Jan 13, 2015 at 16:17
  • $\begingroup$ 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) $\endgroup$
    – user77400
    Commented Apr 15, 2015 at 8:47
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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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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.

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  • $\begingroup$ Your hypothesis is very interesting. My wonder is what's the primary purpose of the body generating electromagnetic signals, I think it's primarily for internal signaling and cell healing, and perhaps externally for danger/presence. Some of us can sense when people are behind us and when we're in danger due to presence of something big. But I don't know of any animals that make use of electromagnetic warning. $\endgroup$ Commented Jul 12, 2023 at 23:29
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A Human could emit just about any frequency of radiation if you get their body hot enough. Black body radiation is based on temperature. Toss someone into molten lava and you might see several different colors.

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What causes humans to emit infra red radiation

According to Wien's displacement law, black body will emit most energy in the wavelengths, described by body temperature relation :

$$ \lambda _{\text{peak}}={\frac {b}{T}}, \tag 1$$

where $b$ is Wien's displacement constant. Calculating that for human body energy emission peak would be at wavelength :

$$ \lambda _{\text{human}}={\frac {2898~\text{μm⋅K}}{36.6~\text{°C}}} \approx 9~\text{μm}, \tag 2 $$

This radiation is long-wavelength infrared, which includes region of EM radiation between $8~–15 μm$, zone which is especially useful for thermal imaging devices.

Hence, the answer is that our body temperature is quite low (comparing to for example Sun surface temperature which is about $5500~°C$), so we can only emit in a moderate amounts photons which are of quite low energy. Btw, that's why we need a thick clothes in a winter season,- for stopping our precious LWIR photons (otherwise body temperature will drop drastically)!

why don't we also emit other types of electromagnetic radiation

We do emit these also. But other types of radiation according to a black-body radiation law will be with a very small intensities (small photon stream). Hence for all pratical purposes you can say it's almost none.

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Humans also emit small but measurable amounts of ionizing radiation. The source of this is mostly radioactive potassium isotopes consumed via foods like bananas.

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  • $\begingroup$ Not just potassium and not just from banana. :-) hps.org/publicinformation/ate/faqs/faqradbods.html $\endgroup$
    – inf3rno
    Commented Jan 1, 2018 at 19:05
  • $\begingroup$ 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? $\endgroup$
    – Slam
    Commented Jan 2, 2018 at 20:33
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    $\begingroup$ Others downvoted it, I upvoted... $\endgroup$
    – inf3rno
    Commented Jan 3, 2018 at 11:03
  • $\begingroup$ I think the down-vote is mostly due to your expression "non-trivial amounts". While in reality so called "banana dose" is a natural background ionizing radiation level which everything around us has, about 0.1 μSv. So your saying "non-trivial" is too much exaggerated. For, example my room now is under radiation level of $0.14~\mu Sv/h$. Same applies to your room or any other standard place on Earth. (This level maybe remnants of cosmic X-rays background and/or Sun field, which is weakened a lot by Earth atmosphere and magnetosphere). $\endgroup$ Commented Dec 3, 2022 at 0:10
  • $\begingroup$ Ah. I meant "non-trivial" as in "measurable" not as in "dangerous" $\endgroup$
    – Slam
    Commented Feb 4, 2023 at 2:12
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There are four laws for thermal radiation among which Kirchoff's law is qualitative and not quantitative so it can't tell about specific frequency or spectrum part. Second is Stefan-Boltzmann law which is purely quantitative so it only tells about total intensity of radiation at given temperature. Stefan-Boltzmann law was used by economic institutes for defining poverty line, amount of energy radiated by a person at body's temperature.

Now Planck's law can gives both but its interpretation and use makes it unsuitable for given purpose. Reason is human body is most of time at thermal equilibirium, and Planck's law can predict visible light which is not. So only Wein's law is suitable and according to which at given temperature radiation of only one frequency is maximum emitted.

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