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

Question

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?

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

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.

Answer 5

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

Answer 6

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.

Answer 7

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.

Answer 8

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.