How electrons absorb or emit photons

[Wille]

TL;DR Summary

My question is about what happens when incoming light hits a solid material and give that material its color. Is it true that those photons which are absorbed (i.e. have enough energy to move an electron in the solid material to a higher level (excite)) are never to be seen again? At the same time I read that electrons always fall back to the ground state and then the photon are emitted again. Which one is it?

Hi,

I am aware that the reason why objects have color is an old subject. However I come across two claims which sound like they are in contrast to each other.
On one hand I read that the photons with specific frequency in incoming light are absorbed (i.e. have enough energy to move an electron in the solid material to a higher level (excite)) and are then never to be seen again. I.e. they are erased from the light spectrum and will never reach my eyes.
On the other hand I read that electrons always fall back to the ground state (they stay excited for a very short time) and then the photons are emitted again. Which one is it?

Could it be that the excited electrons stay excited as long as the light is on?
And what about those frequencies that are not enough to move an electron in the solid material to a higher level, do they have any interaction at all with the electrons? I wonder because I have also read the an electron cannot "reflect" an photon. Only absorb or emit an photon.

Thank you.

 

[RPinPA]

Summary: My question is about what happens when incoming light hits a solid material and give that material its color. Is it true that those photons which are absorbed (i.e. have enough energy to move an electron in the solid material to a higher level (excite)) are never to be seen again? At the same time I read that electrons always fall back to the ground state and then the photon are emitted again. Which one is it?

Both. I'm not sure why you the fact that an electron absorbs a photon contradicts the fact that an electron can emit a photon.

Perhaps it's the phrase the photon that is your problem. The original photon is gone. If the electron goes back to the ground state, a new photon is created with the same energy as the original photon. If the electron falls to some intermediate state, with less of an energy jump, then a new photon with less energy is created.

Or perhaps you're asking why the emission spectrum isn't exactly the same as the absorption spectrum.

The details of this answer depend on exactly what property you're talking about. Let's talk about how things look different colors in sunlight. Sunlight is a broad continuous spectrum created by a process called "blackbody radiation".

In many materials there are lots of those intermediate energy states, especially of molecules in solids. So the excited electron will often transmit its energy to other atoms and molecules in those smaller energy jumps, which are often in the infrared and generate heat. So when light is absorbed by a colored material, the material gets warmer.

What about a laser or an LED? Those are single-wavelength sources whose emission corresponds to some energy jump of the electrons. Some other mechanism is used to excite the electrons, and then they fall down to the ground state, emitting the characteristic wavelength.

 

[Wille]

Ah, thank you for the answer. Yes, I think the line you wrote "Or perhaps you're asking why the emission spectrum isn't exactly the same as the absorption spectrum" is spot on. Now I think I understand. The new photon can be emitted with a frequency belonging to the (for instance) infrared spectrum and that is why I cannot see the emitted photon. I had not considered the intermediate states in solids which you describe. Thank you again, a really great answer.

 

[Wille]

Both. I'm not sure why you the fact that an electron absorbs a photon contradicts the fact that an electron can emit a photon.

Perhaps it's the phrase the photon that is your problem. The original photon is gone. If the electron goes back to the ground state, a new photon is created with the same energy as the original photon. If the electron falls to some intermediate state, with less of an energy jump, then a new photon with less energy is created.

Or perhaps you're asking why the emission spectrum isn't exactly the same as the absorption spectrum.

The details of this answer depend on exactly what property you're talking about. Let's talk about how things look different colors in sunlight. Sunlight is a broad continuous spectrum created by a process called "blackbody radiation".

In many materials there are lots of those intermediate energy states, especially of molecules in solids. So the excited electron will often transmit its energy to other atoms and molecules in those smaller energy jumps, which are often in the infrared and generate heat. So when light is absorbed by a colored material, the material gets warmer.

What about a laser or an LED? Those are single-wavelength sources whose emission corresponds to some energy jump of the electrons. Some other mechanism is used to excite the electrons, and then they fall down to the ground state, emitting the characteristic wavelength.

Again thank you for the answer. Just one more thing: would you say that this

https://www.physicsclassroom.com/class/light/Lesson-2/Light-Absorption,-Reflection,-and-Transmission
is a correct way of describing what is happening? Is it correct to think in terms eigenfrequencies of the electrons?

Thanks.

 

[vanhees71]

For the semiclassical (i.e., classical em. field, quantized matter), see my Insights article on the photoeffect:

https://www.physicsforums.com/insights/sins-physics-didactics/
For a full quantum treatment but in non-relativistic approximation for the matter fields of radiation, see

S. Weinberg, Lectures on Quantum Theory, Cambridge University Press