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Joker93
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I understand why a black body absorbs every frequency(it is the definition of a black body!) but i do not understand why it also radiates at all frequency spectrum.
Hello. No, but my question is why radiating at all frequencies means also absorbing at all frequencies and vice versa. Why does the one mean the other?DaveC426913 said:Is there a reason to expect it would not radiate at certain frequencies?
https://en.wikipedia.org/wiki/Black_body#Cavity_with_a_holeAn approximate realization of a black surface is a hole in the wall of a large enclosure. Any light entering the hole is reflected indefinitely or absorbed inside and is unlikely to re-emerge, making the hole a nearly perfect absorber.
The correct statement is that black body in thermodynamic equilibrium radiates at all frequencies. In principle, a black body might not be in a thermodynamic equilibrium, in which case it might not radiate at all frequencies.Adam Landos said:I understand why a black body absorbs every frequency(it is the definition of a black body!) but i do not understand why it also radiates at all frequency spectrum.
About your last statement, why having maximal entropy means radiating in all frequencies?(i have not taken a course on thermodynamics). Thank you for the answer by the wayDemystifier said:The correct statement is that black body in thermodynamic equilibrium radiates at all frequencies. In principle, a black body might not be in a thermodynamic equilibrium, in which case it might not radiate at all frequencies.
If you want to ask why thermodynamic equilibrium implies radiation at all frequencies, it is because thermodynamic equilibrium, by definition, is a state with maximal entropy.
Entropy can be thought of as the number of different possible states. Clearly, there are more possible states if all frequencies are allowed than if only some of them are allowed.Adam Landos said:About your last statement, why having maximal entropy means radiating in all frequencies?(i have not taken a course on thermodynamics). Thank you for the answer by the way
Black bodies radiate in all frequencies because they are perfect absorbers and emitters of electromagnetic radiation. This means that they absorb all incoming radiation and then emit it back out in all frequencies, making them appear black to the human eye.
The frequency spectrum of a black body is directly related to its temperature. As the temperature of a black body increases, the frequency and intensity of the radiation it emits also increases. This is known as Planck's law of black body radiation.
A black body is considered a theoretical concept because it does not exist in nature. It is an idealized object that has perfect absorption and emission properties, which cannot be achieved in reality.
The Stefan-Boltzmann law states that the total energy emitted by a black body is proportional to the fourth power of its absolute temperature. This means that as the temperature of a black body increases, the total amount of radiation it emits also increases significantly.
Black body radiation is significant in understanding the universe because it is one of the key pieces of evidence for the Big Bang theory. The current temperature of the cosmic microwave background radiation, which is a form of black body radiation, supports the idea that the universe began with a hot, dense state and has been expanding and cooling ever since.