How Do Absorption/Emission Lines Relate to Black Body Radiation?

[Drakkith]

I've been reading a book a Quantum Physics and I had a question. The book talks about absorption and emission lines in spectroscopy and why they happen. I was wondering if there was any relation between these and to black body radiation. Heating an object causes it to glow a certain color based on it's temperature, but I've read that different elements will emit different color light, such as the different colors in arc lamps or neon signs. What is the difference in the two?

 

[Andy Resnick]

Thermal radiation (black body radiation) does not depend on any material- it is the spectrum of a radiation field at thermal equilibrium (with some temperature).

Absorption/emission lines are due to specific transitions between two quantum states of an atom/molecule. Generally speaking, electronic transitions result in visible light, vibrational transitions result in infrared, and rotational transitions are in the microwave region. Ionization is associated with x-rays.

 

[Dr Lots-o'watts]

May I add that the reason why black body radiation is a continuum (rather than discrete, distinguishable transitions) is that it is produced by classical, macroscopic systems containing innumerable overlapping quantum states which can't be resolved (as expected, classical macroscopic energies are not quantized in levels).

For this reason, a star such as the sun gives off mostly a black body spectrum. But since it's content in atomic gases (such as hydrogen) is so great at the surface, some discrete transitions can be identified as dents in the black body continuum.

The first graph in this link shows the sun's general blackbody emission, overlapped with discrete absorption lines ("Fraunhofer lines") from its surface gases:

http://www.newport.com/store/genContent.aspx/Introduction-to-Solar-Radiation/411919/1033

 

[Andy Resnick]

May I add that the reason why black body radiation is a continuum (rather than discrete, distinguishable transitions) is that it is produced by classical, macroscopic systems containing innumerable overlapping quantum states which can't be resolved (as expected, classical macroscopic energies are not quantized in levels).

For this reason, a star such as the sun gives off mostly a black body spectrum. But since it's content in atomic gases (such as hydrogen) is so great at the surface, some discrete transitions can be identified as dents in the black body continuum.

The first graph in this link shows the sun's general blackbody emission, overlapped with discrete absorption lines ("Fraunhofer lines") from its surface gases:

http://www.newport.com/store/genContent.aspx/Introduction-to-Solar-Radiation/411919/1033

I understand what you mean, but that's not exactly right- the absorption/emission spectrum of macroscopic bodies *in general* is given by the large number of degrees of freedom/available quantum states. Blackbody radiation, as an abstracted idealized concept, is not a function of any material object, and is in fact unrelated to massive objects.

 

[Dr Lots-o'watts]

Blackbody radiation, as an abstracted idealized concept, is not a function of any material object, and is in fact unrelated to massive objects.

It applies to something else besides massive objects?

 

[Drakkith]

Thermal radiation (black body radiation) does not depend on any material- it is the spectrum of a radiation field at thermal equilibrium (with some temperature).

Absorption/emission lines are due to specific transitions between two quantum states of an atom/molecule. Generally speaking, electronic transitions result in visible light, vibrational transitions result in infrared, and rotational transitions are in the microwave region. Ionization is associated with x-rays.

I've read about light being given off when electrons jump up or down states, but how do the other transitions give off radiation?

 

[Andy Resnick]

It applies to something else besides massive objects?

Blackbody radiation has nothing to do with massive objects. As I stated above, "blackbody radiation" refers to the spectral distribution of an electromagnetic field in thermal equilbrium with a reservior at some temperature T. If you like, the field is equivalent to a gas of photons at some temperature T:

http://en.wikipedia.org/wiki/Photon_gas

 

[Andy Resnick]

I've read about light being given off when electrons jump up or down states, but how do the other transitions give off radiation?

Because the different states are at different energies- this is the first half-way useful link I found:

http://www.chem.ufl.edu/~itl/4411L_f00/hcl/hcl_il.html

 

[Drakkith]

Because the different states are at different energies- this is the first half-way useful link I found:

http://www.chem.ufl.edu/~itl/4411L_f00/hcl/hcl_il.html

Ah, I understand a bit better now. Thanks Andy. =)