How does blackbodies radiation work? I understand from Maxwell's equations that electromagnetic radiation occurs when there are accelerating charges. Some examples are rotating or stretching of electric dipoles (which is maybe the case for some molecules?). It seems that many electrically neutral objects are good approximations of a blackbody. What if we consider a (uniformly) charged medium instead? Would the spectrum still be the same as neutral blackbody radiation?
Asked
Active
Viewed 504 times
0
-
A "black body" is a huge source of all kinds of waves. It is not transparent for an incident wave - thus the name a "black body". Thanks to its huge size, it radiates a unique spectrum only depending on the body temperature. And there is a tail of the (hignh frequency) spectrum that is quantum, not classical. – Vladimir Kalitvianski Jan 21 '22 at 16:42
-
Perhaps these will help: How does radiation become black-body radiation?, Black body vs. Thermal radiation – Roger V. Jan 21 '22 at 16:47
2 Answers
1
Without going into the subtle details of quantum mechanics (which is the right theory to understand blackbody radiation), classical physicists already understood thermal radiation as coming from the motion of the particles that compose matter. Matter is made from charged particles and thermal effects lead to the random jiggly motion of these particles (accelerations and decelerations), thus producing thermal electromagnetic radiation.
Not all electrically neutral objects can be a blackbody as, by definition, a blackbody emits and absorbs "equally" in all wavelengths. Objects that seemingly do not absorb in the optic range (like glass) may absorb pretty significantly more in the ultraviolet range. A metal may be transparent to UV but it is clearly reflective in the optics range. The composition of the material matters a lot.
Similarly, the spectrum of the uniformly charged medium that you mentioned will depend a lot on its composition.
Mauricio
- 5,273
1
Mechanisms of radiation emission
Classically electromagnetic radiation is created by accelerating charges. From the quantum point of view, light is emitted when electrosn transition from higher to lower energy levels. These are rather general statements - precise mechanisms depend on the material that we are discussing.
Black body radiation
Black body radiation is not a particular mechanism, but radiation is thermal equilibrium (i.e., describe dby Boltzmann distribution). Black body is a theoretical concept - a body that can emit and absorbs radiation at all frequencies, and which absorbs all the radiation incident on it (i.e., there is no reflection).
Electrical neutrality
Object may contain accelerated charges and still be electrically neutral. Electrical neutrality means only that the amount of positive and negative charge in the object is equal, but not how this charge moves.
Thermal radiation
Radiation emitted by objects heated to a certain temperature is often well described by the black body spectrum. This has nothing to do with whether such an object is electrically neutral or not.
Roger V.
- 58,522