Why don't absorption and emission spectra "cancel out"?

[marksyncm]

My question is regarding absorption and emission lines.

As far as I understand, when a photon of the "right" wavelength passes through an atom, an electron is excited and takes on a higher energy state, in proportion to the energy imparted by the photon. This is the "absorption" part. However, the electron will rapidly de-excite, returning to its previous energy state and emitting a photon in the process. This is the "emission" part.

My assumption: the emitted photon is of the exact same wavelength as the absorbed one; if a photon of wavelength X was absorbed, then a photon with the exact same wavelength X will be emitted.

If my assumption is true, then how is it that we can tell that absorption happened? If a photon of wavelength X is absorbed and then a photon of wavelength X is emitted, it's as if no absorption took place at all and it would seem to me no absorption lines should be visible on spectroscopic examination.

What am I missing?

Thanks.

 

[phyzguy]

It's a complicated question, but the following is a simple answer. Imagine light from a distant light source (like a star) passing through a cloud of gas consisting of atoms in the ground state. An atom in the gas cloud absorbs a photon from the star, and then some time later re-emits it. The light which is re-emitted can be re-emitted in any direction, so the probability that it is re-emitted in the same direction as the original photon is very small. So you see a reduction in the number of photons that reach you, which is an absorption line.

 

[marksyncm]

Thank you phyzguy, this makes sense.

 

[marksyncm]

(Sorry for the double post; not sure if anyone would notice an edit at this point.)

Does this mean that when looking towards seemingly "empty" regions of space (meaning a region where we do not see a star), we should expect to find hydrogen emission lines from the photons that were scattered under random "angles" by hydrogen atoms inside random stars?

 

[CWatters]

(Sorry for the double post; not sure if anyone would notice an edit at this point.)

Does this mean that when looking towards seemingly "empty" regions of space (meaning a region where we do not see a star), we should expect to find hydrogen emission lines from the photons that were scattered under random "angles" by hydrogen atoms inside random stars?

Light scattered by an object appears to come from the object. So light "scattered under random "angles" by hydrogen atoms inside random stars" would appear to come from those stars. We only see light scattered in our direction.

We do see hydrogen elsewhere..

https://uanews.arizona.edu/story/hydrogen-hydrogen-everywhere

 

[marksyncm]

Thank you.

 

[DrDu]

Another aspect is that emission and absorption cancel out in thermal equilibrium. E.g. if you look into a hot ceramic oven, you won't be able to distinguish the pots inside when there is no external light source although the light from the glowing pots inside may be very intense.

 

[Henryk]

However, the electron will rapidly de-excite, returning to its previous energy state

Actually, electron doesn't always return to its previous state. In many cases it loses its energy to a different energy level than the original one. Then it can decay to its original (presumably ground) state emitting a photon of a different energy. This is known as luminescence.
It can also decay by a non-radiative process (e.g. collisions with other atoms and/or molecules, if an atom is a part of solid or liquid, the energy can be lost due to thermal motion).
Second thing, the loss of energy doesn't always happen quickly. Some excited states have lifetime of the order of hours !.
There are other processes possible between photons an atoms (liquids, solids).
One is a scattering, i.e. simultaneous absorption of the incoming photon and emission of another photon. This could be elastic scattering, that is the outgoing photon of the same energy as the original, (that's what make the sky blue! and sunset orange-red) or inelastic scattering (Raman, Rayleigh). And, of course, stimulated emission which happens when the atoms was originally at a higher energy level - this is how lasers work.
The most common is coherent elastic emission of a photon of the same energy but differing phase. This is what gives rise to things like refractive index and mirror-like reflection.

 

[marksyncm]

Thanks DrDu and Henryk, this was very helpful.