# Question about Planck's Law and temperature/light emission

1. Sep 5, 2011

I have a question about Planck's Law. When I first read about it, I misunderstood it to mean that an object at a certain temperature would only emit a very narrow wavelength of light. But as I've looked into it further it appears as though everything in the universe emits a range of light that looks like a bell curve, and Planck's Law only specifies the peak wavelength of a black body at a given temperature. Right?

Okay, so check out this graphic of black bodies of various temperatures (this is a pretty typical graphic used in black body description pages on the Internet):

It would appear to imply that everything in the universe emits all wavelengths of light; that even the weakest parts of an object's spectrum are still not zero. If that's the case, then can I assume that human beings must emit visible light (in addition to their IR peak)? I know the retina is sensitive enough to respond to a single photon, but it appears that 5 - 9 photons are required within a 100ms or so to actually register a response in the brain. So are all the people around me giving off visible light that I'm not seeing?

If so, that's awesome. If not, what am I missing?

Last edited by a moderator: Apr 26, 2017
2. Sep 5, 2011

### Mordred

I'm not too familiar with Plancks law but I can adress one of your questions. The human eye cannot see all spectrums of light only those spectrums that count as visible light those do do include gamma, xray etc. Also we cannot see frequency changes past 66 hertz I bleive is the correct value. A good example of this is to wave your hand rapidly in front of your eyes rabidly you won't see the full movment only points of that movment. TV makes excellent usage of this in animation.
Everything in nature does radiate as all matter has a half life I can't recall the correct form of that radiation however.

3. Sep 5, 2011

Thanks Mordred, I'm already aware of everything you said. My understanding is that the peak emission of a typical human is in the infrared - but do we give off small quantities of visible light? And even more generally... does every object in the universe give off all frequencies of light, just at differing intensities?

4. Sep 6, 2011

Staff Emeritus
Yes, but it's incredibly dim. As in a couple of photons per day.

5. Sep 6, 2011

### Ken G

Also, the Planck spectrum is a kind of idealized emission, based on the assumptions of complete thermodynamic equilibrium (so a single temperature) and that the object is a "blackbody" (absorbs all light that impinges on it, and emission of light is just the inverse process to that). Neither of those are ever perfect assumptions, but they're often not too bad. More to the point, they represent such a huge simplification of a very difficult problem that we tend to squint our eyes and try to apply them even when they don't strictly apply, as for a human being.

6. Sep 6, 2011