# Blackbody radiation

#### viona

Hi every one
I have three simple questions: what causes Blackbody radiation? what does Planck meant by the oscillators? why the Blackbody radiation is continuous while the emission spectra of atoms (gases) is discrete?

I know that the discrete emission spectra of atoms was explained by Bohr's theory: the electrons in the atoms are in orbits with different energies and when an electron goes from higher energy orbit to a lower energy orbit it emits an electromagnetic radiation with specific frequency.

Classical theory tried to explain thermal radiation in condensed matter (continuous spectrum) by assuming that the thermal radiation originates from accelerated charged particles in the atoms near the surface of the object; those charged particles emit radiation much as small antennas do. Did they mean that these charged particles are the electrons? was this a correct idea?

Planck explained it by assuming that there are atomic oscillators and they are quantum oscillators . What are these atomic oscillators? they are the electrons inside the toms? they are atoms oscillating due to the thermal energy? or both of them?

Does the continuous spectrum from blackbody radiation is caused by electron transitions from one orbit to another inside the atom or by the oscillating atoms due to the thermal energy?

#### topsquark

Forum Staff
A blackbody is one that emits all the energy that it absorbs. We have not found any material that actually does this but we can get clever. Planck used some material (I don't know what it was made of. He probably didn't specify one...this is a theortical calculation after all.) with a cavity inside and a small hole in one point. Ideally all of the radiation coming into the hole would "bounce around" inside the cavity until it happened to come back out the hole. It is the actual hole, not the material or the cavity that is the black "body."

The blackbody spectrum was a confused mess until Planck came along. Typically the spectrum was calculated and it turned out not to have the same results as experiment. Using Classical theory you would set up an integral. Planck used a technique called Statistical Mechanics which was a new thing and not entirely trusted. The main idea was that you break down the radiation into little pieces and instead of integrating you would sum up all the little pieces, then you would calclulate the limit as the pieces get infinitesimally small. This produced the same problems as the Classical way of calculation. But then a weird thing happened. Planck discovered that if didn't take the limit of the little pieces the spectrum his calculation did come up with matched the experimental values. We woiuld now-a-days call the little pieces "quanta" or, later on, photons.

Planck wasn't able to explain why this worked and figured the quantum aspect was caused by something in the material that was oscillating to produced the spectrum rather than in the radiation. Einstein was one of the first to take the idea seriously and was able to prove that it was the radiation itself that was quantized when he wrote his landmark paper on the photoelectric effect.

Theoretically the spectrum will be continuous. At certain frequencies the radiation that gets absorbed by the atoms will eventually re-emit the radiation. If we don't hit the frequeny that an atom would absorb then the radiation is not absorbed but will "bounce around" inside the cavity and eventually come back out. But that came later when the structure of the atom was discovered and how the electrons bounce around in the atomic energy levels when absorbing radiation of specific frequencies. Remember, though, the structure of atoms wasn't known when Planck did his work. As a result he didn't worry about the structure of the material and how the radiation was absorbed and emitted, he just assumed that the radiation was not quantized.

-Dan

viona

#### studiot

Hello viona and welcome.

Black body radiation is thermally generated radiation.

To answer your questions, let us start a bit further back.
Heat can be transferred by conduction, convection or radiation.

with Stephan's law, which you may have heard of.

viona and topsquark

#### studiot

Thanks for the vote topsquark, though my post doesn't merit it as it stands.

I lost the best part of an hour's thinking, composing and typing on this one, all due to an unplanned disconnection.
I don't suppose there is any way to recover it, it was in the box?

#### topsquark

Forum Staff
Thanks for the vote topsquark, though my post doesn't merit it as it stands.

I lost the best part of an hour's thinking, composing and typing on this one, all due to an unplanned disconnection.
I don't suppose there is any way to recover it, it was in the box?
I can't say for sure but I doubt if it can be done. I know there was a way (with the old software) to restore a deleted post but I doubt you'll get that from the disconnect. And I don't know if this version of the software will even let you restore a deleted post

Sorry!

And, no, you deserved the Like. Your post didn't give any details but it was completely relevant..

-Dan

#### viona

A blackbody is one that emits all the energy that it absorbs. We have not found any material that actually does this but we can get clever. Planck used some material (I don't know what it was made of. He probably didn't specify one...this is a theortical calculation after all.) with a cavity inside and a small hole in one point. Ideally all of the radiation coming into the hole would "bounce around" inside the cavity until it happened to come back out the hole. It is the actual hole, not the material or the cavity that is the black "body."

The blackbody spectrum was a confused mess until Planck came along. Typically the spectrum was calculated and it turned out not to have the same results as experiment. Using Classical theory you would set up an integral. Planck used a technique called Statistical Mechanics which was a new thing and not entirely trusted. The main idea was that you break down the radiation into little pieces and instead of integrating you would sum up all the little pieces, then you would calclulate the limit as the pieces get infinitesimally small. This produced the same problems as the Classical way of calculation. But then a weird thing happened. Planck discovered that if didn't take the limit of the little pieces the spectrum his calculation did come up with matched the experimental values. We woiuld now-a-days call the little pieces "quanta" or, later on, photons.

Planck wasn't able to explain why this worked and figured the quantum aspect was caused by something in the material that was oscillating to produced the spectrum rather than in the radiation. Einstein was one of the first to take the idea seriously and was able to prove that it was the radiation itself that was quantized when he wrote his landmark paper on the photoelectric effect.

Theoretically the spectrum will be continuous. At certain frequencies the radiation that gets absorbed by the atoms will eventually re-emit the radiation. If we don't hit the frequeny that an atom would absorb then the radiation is not absorbed but will "bounce around" inside the cavity and eventually come back out. But that came later when the structure of the atom was discovered and how the electrons bounce around in the atomic energy levels when absorbing radiation of specific frequencies. Remember, though, the structure of atoms wasn't known when Planck did his work. As a result he didn't worry about the structure of the material and how the radiation was absorbed and emitted, he just assumed that the radiation was not quantized.

-Dan
Thank you for your explanation. I understood from your answer (among other things) that when Planck did his work the structure of the atoms wasn't known. But today if we want to explain the reason of the thermal radiation (from solids or liquids) can we explain it by:
1- the electrons which due to thermal energy go to excited states and then go to lower energy states while emitting radiation,
or by:
2- the vibration (and/or rotational motion) of atoms (or molecules),
or by both of them?

#### Woody

Yes, both.
Different frequencies of radiation will match the resonant frequencies of different structures within a material.
Higher frequencies match the resonant frequencies of the electrons in the atoms, (different frequencies for different elements and in different chemical bonding environments).
Lower frequencies (micro-waves) match the resonant frequencies of of the vibrations and rotations of the bonds within molecules.

viona and topsquark
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