Formation of emission lines and other topics

[Hak]

If we spectroscopically observe a cloud of hot gas, which is on the whole not very absorbent, and which is not illuminated by a source behind it, we observe emission lines. How does this type of spectrum form?

I had thought that those lines are those in which there are transitions of atoms is true, but I don't think that's enough. Why should all the material as a whole emit like that. Why are there these electronic transitions?

And then: if there is a light source behind the material, one observes a spectrum that is in a way the negative of the one I put, i.e. the lines become absorption lines. I had thought that something different must be happening in the two cases, although I don't know what.

And then again: if we knew nothing about electronic transitions and only wanted to consider the macroscopic properties of the gas (which could be composed of complicated molecules, in which there are not only electronic transitions but also other phenomena), could we still justify the fact that the emission and absorption spectra are the negative of each other?

Thank you for any input; complex, articulate and in-depth answers are also welcome.

As usual, please feel free to move the thread to the forum you deem most appropriate. Thank you.

 

[phyzguy]

If you have a cloud of hot gas, some atoms are in high energy excited states. When the atoms transition to lower energy states, they emit light of a certain wavelength. When many atoms do this, we see the light as a bright emission line. "all the material as a whole" doesn't emit light. Each atom either transitions to a lower state or not, but in a cloud of gas there are many atoms, so there are many atoms emitting light.

If you have a light source behind the cloud of gas, there are some atoms in the gas that are in lower energy states. These atoms absorb the light from the background source and transition to a higher energy state. This removes some of the light from the background source, so we see a dark absorption line.

If the lower and higher energy states are the same in the two cases, then the wavelength of the bright emission line and the dark absorption line are the same.

 

[Hak]

If you have a cloud of hot gas, some atoms are in high energy excited states. When the atoms transition to lower energy states, they emit light of a certain wavelength. When many atoms do this, we see the light as a bright emission line. "all the material as a whole" doesn't emit light. Each atom either transitions to a lower state or not, but in a cloud of gas there are many atoms, so there are many atoms emitting light.

If you have a light source behind the cloud of gas, there are some atoms in the gas that are in lower energy states. These atoms absorb the light from the background source and transition to a higher energy state. This removes some of the light from the background source, so we see a dark absorption line.

If the lower and higher energy states are the same in the two cases, then the wavelength of the bright emission line and the dark absorption line are the same.

Thank you very much. Could you explain better, though, why a molecule tends to emit more at the frequencies where it absorbs more? Thank you again.

(P.S. Is this the correct Forum for such a topic or is there a more specific one? I'm afraid the topic may not interest many people...)

 

[phyzguy]

Generally we're talking atoms here, not molecules. Molecules are a lot more complicated. Electrons in atoms have discrete energy levels. When the electron moves from state A (lower energy) to state B (higher energy), it absorbs a photon of a specific energy. When it drops down from state B back to state A, it emits a photon of that same energy. That's how atoms behave. Look up any book on atomic physics.

 

[Hak]

Generally we're talking atoms here, not molecules. Molecules are a lot more complicated. Electrons in atoms have discrete energy levels. When the electron moves from state A (lower energy) to state B (higher energy), it absorbs a photon of a specific energy. When it drops down from state B back to state A, it emits a photon of that same energy. That's how atoms behave. Look up any book on atomic physics.

Thank you. Initially, I thought of a recently tried experiment. Let's take two very small balls and connect them to a spring. This system is a molecule.

If the molecule is alone, its frequency is the only noticeable one. In fact, the spring+balls system oscillates with a typical frequency and every so often there is a change in its energy. The radiated energy that comes out, the only one there is, is its own light (e.g. our molecule could have red as its light).

Let us now suppose that the molecule is oscillated by many kinds of oscillations (i.e. it is illuminated from behind).
All the oscillations, except the one of the same frequency as the molecule, will be partially absorbed by it then their residual energy will continue on its way.
Where does the energy that the molecule in this case absorbs go? Macroscopically, do we see an increase in the temperature of the gas?
If we observed the system for a very long time we would see that eventually the light disappears.
The only explanation I know of is that a system always tends to have as many configurations as possible, i.e. its entropy increases. And when you have approached the state of equilibrium there is no longer any exchange of energy. This is what is also called thermal death.

Let us imagine that our universe consists of the system in the figure. As mentioned, light eventually disappears, everything is still, and energy, in this case only potential, is minimal. Where does all the other energy go? Did the system responsible for transporting the radiated energy take it by any chance?

I have the impression that this is definitely not how it works at the quantum level and that this cannot be modelled classically. What do you say?

 

[phyzguy]

This is not the way to learn physics. If you really want to know the answers to your questions, get a book on atomic physics and study it. I'm signing off of this thread.