Quantum Physics Question (6.5)

[SwedishPersian]

"An absorption spectrum is formed when photons of specific wavelength are absorbed by the electrons of an atom, which then make transitions to higher energy levels. But as soon as the electrons reach the higher energy level they will fall back to the state they came from, re-radiating photons of precisely the same wavelength as those they absorbed. So there should not be any dark lines. What do you say?"

I hate how there is no mark scheme in the book for "word answer" questions. So I was wondering if anyone here could answer this.

Thanks

EDIT: Here is another annoying one, I think it has something to do with the Heisenberg uncertainty principle:

"An electron of kinetic energy 11.5 eV collides with a hydrogen atom in its ground state. With what possible kinetic energy can this electron rebound off the atom?"

Edited January 1, 2010 by SwedishPersian

[sweetnsimple786]

What I'm about to say comes from my scant knowledge of chemistry, so beware, I guess. haha

For the absorption spectrum, the dark lines are the photons that have been absorbed by the thing behind the actual spectrum. The colors are the ones that are reflected back or not absorbed. It's the opposite for the emission spectrum. There's probably more to this explanation, but that's what I understand.

[SwedishPersian]

What I'm about to say comes from my scant knowledge of chemistry, so beware, I guess. haha

For the absorption spectrum, the dark lines are the photons that have been absorbed by the thing behind the actual spectrum. The colors are the ones that are reflected back or not absorbed. It's the opposite for the emission spectrum. There's probably more to this explanation, but that's what I understand.

yeah that right, but why do the dark lines still appear if the electron re-emits the absorbed photon?

[sweetnsimple786]

I might be misunderstanding it, but the electron re-emits it and jumps to a lower energy level. but then that re-emitted photon is absorbed by the gas.

Am I reading it wrong?

[SwedishPersian]

I might be misunderstanding it, but the electron re-emits it and jumps to a lower energy level. but then that re-emitted photon is absorbed by the gas.

Am I reading it wrong?

how do you mean "absorbed by the gas"

[sweetnsimple786]

I can't recall exactly, but the way the absorption spectrum works is there's something between the object that shows the dark lines and the light source. That thing is a gas and it does the absorption/emission. When you have continuous light, you're going to have this continuous emission and absorption by the gas that leads to the lines. Once again, that might not be right, but that's what I recall from a chem class last year.