Actually this inquiry is mercifully brief. Infrared radiation, a narrow element of the whole EM spectrum, is thought to be the one that can impart its energy in the form of heat. Or is that merely the root cause of emission of such wavelengths? Does heat develop at the place the photons are absorbed? And if so, then is this power constrained to only a narrow lower band of the EM frequency spectrum? Could infrared radiation in sufficient amount set a piece of paper on fire? Could higher frequencies do that? Which frequencies, and why?
Here's a Wiki article on infrared heat radiation. Basically, any electromagnetic radiation, no matter the frequency will transfer energy to an object that absorbs it. Some frequencies transfer heat better because they are less likley to be reflected. Heat in an object is the movment of molecules in the object. Quote from the Wiki
I'll answer as I understand things, and I'm hopeful that James of DH will come along any messes I create. This will be a bit of an expansion on Verns response. The first point is that we must remember to distinguish between the heat that occurs by thermal coupling of two systems, and thermal radiation from one system to another. So, when my girlfriend sticks her freezing cold feet on my warm back in the middle of the night, heat is transfered from my body to hers by thermal conduction. Likewise, when the sun warms the surface of the earth, it does so by electromagnetic radiation. Now, I think of ``heat'', in the sense that you are using it, as a bulk phenomenon. That is, at the atomic level, ``temperature'' is just velocity---for example, the temperature of a gas is related to it's average velocity squared: \(T \sim \bar{v}^2\) The \(\bar{v}\) is the average velocity of a gas molecule. So, lets simplify things. How would you heat up a bunch of gas? The obvious answer is ``make all the molecules move faster''. This can be done in one of two ways: 1.) Bring another, hotter (i.e. more energetic) system into thermal contact with it. This can be done by mixing in a warmer gas. The picture you can imagine is that the ``cold'' gas atoms are moving lazily around. The ``warmer'' gas atoms are introduced into the system, and are speeding around the container, bumping into each other, and the cold gas atoms at the same time. Because the collisions are more or less elastic, the colder atom gets some of the hotter atom's energy after the collision, which means the temperature equalizes, eventually. 2.) You can irradiate the gas with some radiation. This means sending a bunch of photons into the container, which collide with all of the slow moving gas atoms, making them move faster. So what tells us that this latter process has to be IR radiation? It is my understanding that nothing does! That is, any radiation has the potential to make the gas more energetic. (Wikipedia confirms this, if you trust them.) I think IR could be used to set paper on fire. A Youtube video confirms this! http://www.youtube.com/watch?v=KWmdvhaRGwU But you can also burn paper with other wavelengths, too. Here's a green laser doing the job: http://www.youtube.com/watch?v=te7rS3-4unY
Thanks, the above is good and helpful. At the root of my curiousity, I admit now, is my wondering if ordinary visible light, when greatly blue-shifted toward higher energy (perhaps by one or another set of cosmic circumstances) -- would THAT tend to set fire to things?? I guess the clear answer is yes, and that ALL EMR has the potential to create heat at the place of absorption. But does blue-shifting ordinary light (which drives it further away from the infrared band) make it more, or less, likely to impart heat?
When ordinary light is Doppler shifted toward the blue, some frequencies move out of the IR range and a previously lower frequency moves in. So I suspect you wouldn't see much change. The infra red range seems good at setting molecules in motion. Higher frequencies may be more easily reflected. Bathed in the same light, a black piece of paper will warm faster than a mirror, for example.
Hi Ben: Nothing really wrong in your post 3, but some things might confuse. (1) Your girl friend's cold feet on you warm back are also being warmed a very slight amount by radiant energy transfer. I.e. just because there is solid to solid contact does not stop radiative transfer. To make the point more clearly, radiation does NOT require a radiating surface but is also an internal process. I some glasses at some temperatures the internal transfer by radiation form one molten part to a still solid part being melted is actually greater than the thermal conduction transfer. Glass is not good thermal conductor but if IR dark it is a good internal radiator and absorber. I believe that radiative transfer dominates conduction all most everywhere within the sun (assuming one distinguished between conduction and convection. In the convection zone, that is dominating means of heat transfer.) (2) Your "temperature'' is just velocity” is very misleading and not because of the \(T \sim \bar{v}^2\) For example a gas of hydrogen with the same velocity distribution as an oxygen gas is NOT at the same temperature. O2 has atomic weight of 32 and H2 of 2 so O2 is 16 times more massive. If H2 gas has the same temperature as O2 then the H2 molecules are going 4 times faster. \(T \sim \bar{KE}\) where KE is the kinetic energy. Twice an H2 molecule’s KE = mV^2 and for O2 molecule its twice KE is Mv^2. For these to be equal, given that M/m = 16 we have V/v = 4. I am sure you know all this, but need to be more careful not to state thing incorrectly to one who does not already know. You probably also know that stars are plasmas and the free electrons are much lighter than the ions, so they must move much faster than the ions as both are locally at the same temperatures; however, the electron velocity cannot exceed the speed of light. I forget the details but think this determines why only stars more massive than 1.4 times the sun may become neutron stars.
Thanks! In white light the vacated IR range is supplanted, so little net change. Now, what if you started with separated light already higher in frequency than IR, such as green et al, and greatly blue-shifted it. You say it would be more likely to be reflected, and that to me makes some sense: IR range comprises some ideal/fertile zone for enticing heat at an absorber or has some magic mojo of sorts? You mentioned reflection but if it does get absorbed, photon energy can manifest in other than heat? such as restructuring maybe? like maybe as in photovoltaic material? But heat is the ordinary response? Ordinary also in the case of my example, greatly blue-shifted green (and on up) light? [Edit: Ben mentioned gas as a super-easy example for the absorbing medium, but I'm more interested in solids, such as paper and stuff]
