# Help Reviewing for exam

#### fball558

1. The problem statement, all variables and given/known data
i am reviewing for a physics exam tomorrow. they posted practice problems and with answers but dont show you how to get the answers. i got most the problems but there are a few im not sure so figured i would come here and see if you guys can explain it to me.

the problem is as follows

Problem 3: A certain metal is kept at a very low temperature. It is observed that when
photons with energies between 0.2 and 0.9 eV strike the material, only photons of 0.3 eV,
0.4 eV and 0.7 eV are absorbed. If the metal is heated to the point that 0.7 eV photons are
emitted, what photon energies would you expect to observe?

A. 0.3 and 0.7 eV
B. 0.1, 0.3, 0.4, and 0.7 eV (correct answer)
C. 0.3, 0.4 and 0.9 eV
D. 0.2, 0.4, and 0.7 eV
E. 0.7 and 0.9 eV

i dont even know where to start or how they got this answer. second line syas something about 0.2 - 0.9 eV and the answer has a 0.1??

#### Mzachman

I have the same problem and am wondering the same thing... I don't remember going over this...

Are you in 172?

#### fball558

yeah, got to love 172.
i agree i dont remeber covering it. hopefully it will not be on the exam but i figured i would try and understand the problem just in case.
good luck tomorrow night on the exam.

#### Mzachman

OH, I think I just got it. So by saying "only photons of 0.3 eV, 0.4 eV and 0.7 eV are absorbed" they are saying those are energy level differences between energy level 1, and levels 2, 3, and 4. (1 to 2 is .3 eV, 1 to 3 is .4 eV, and 1 to 4 is .7 eV). So when the .7 eV photons are emitted then they should all be coming back out, so it's the reverse process.

The difference from 4 to 3 (and 2 to 1) is .3 eV, 4 to 2 (and 3 to 1) is .4 eV, 4 to 1 is .7 eV, 3 to 2 is .1 eV. That takes care of .1, .3, .4, and .7

Somebody please let me know if I'm right though. That is just how I'm seeing it. Thanks

#### Mzachman

Thanks about the good luck thing btw, you too.

Just curious, do you have Csathy?

#### fball558

ok i see where your going. kinda makes since, but what does the heat have to do with anything? is that what would make it jump to a lower energy level?

#### fball558

yeap csathy at 1:30 T and TH

#### Mzachman

Yeah, because if you look at the lecture notes the warmer temps drop out of the higher energy states.

Population of level: ~ exp(−E / kT )

That says that if it's really cold there is is a much higher probability of it being in the higher energy level. When it warms up the population level drops.

HA, same here. Exact same class. Small world.

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