Why Are Electric Dipole Selection Rules Confusing on the GRE?

[yosofun]

Hi, I have a question on selection rules for electric dipole transitions. It has to do with a GRE Physics exam question that's confusing a number of students at grephysics.net ... if any of the quantum guru's here would like to help, please check it out at http://grephysics.yosunism.com/disp.php?yload=prob&serial=1&prob=92

thank you.

 

[CarlB]

The problem has to do with an excited atom that emits a photon. Spin doesn't contribute to the energy and so cannot provide a reason for a photon to be emitted. Intuitively, spin just doesn't get you going in the direction you need to go.

This wouldn't be the case if you were in a strong magnetic field, by the way.

Carl

 

[yosofun]

what about delta j and delta n?

The problem has to do with an excited atom that emits a photon. Spin doesn't contribute to the energy and so cannot provide a reason for a photon to be emitted. Intuitively, spin just doesn't get you going in the direction you need to go.
This wouldn't be the case if you were in a strong magnetic field, by the way.
Carl

 

[CarlB]

Since j and n do contribute to the energy (unlike spin).

The reality is that there are always going to be some problems on the GRE that you're just not going to get right because the authors were too esoteric in their problem writing. There's probably some text out there that stresses this particular analysis.

The sad fact is that there are many analyses of basic physics in standard textbooks that are completely wrong. This doesn't stop them from asking you questions about it. But in this case, I think the answer is correct, but the problem is unnaturally difficult.

Carl