Can electrons absorb a photon?

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  • #1
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I was watching this video ( ), and around the 1:52 mark the woman said that it is impossible to image molecules with visible light. By her demonstration, I took this to mean that we can't use visible light to image molecules because visible light is too large to be reflected by the molecule. The problem that this raised in my mind is how can electrons reflect light if a molecule, which is much larger than an electron, cant? If anyone has an answer, I would be glad to hear it.
 

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  • #2
phinds
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  • #3
jbriggs444
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Electrons, per se, do not absorb (or emit) photons either. From Wiki: "An isolated electron at a constant velocity cannot emit or absorb a real photon; doing so would violate conservation of energy and momentum."

They can only absorb a photon in context. For instance, an electron in an orbital in an atom can absorb a photon and move to a higher energy orbital. Now was it the electron that absorbed the photon or the atom that did so?
 
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  • #4
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I guess my follow up then is how can electrons absorb beams of light that are much bigger than them.
 
  • #5
jbriggs444
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I guess my follow up then is how can electrons absorb beams of light that are much bigger than them.
What makes you think that they can?

Edit: there is some dissonance here. A "beam of light" and a "photon" are not the same thing.
 
  • #6
Dale
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The problem that this raised in my mind is how can electrons reflect light if a molecule, which is much larger than an electron, cant?
Neither electrons nor atoms nor molecules reflect light. Surfaces reflect light, meaning a large number of molecules that are fairly smooth on the scale of a wavelength of visible light. Individual electrons can only scatter light. Atoms and molecules can additionally absorb or emit light.
 
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  • #7
Drakkith
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Electrons don't reflect photons, they absorb them, go to a higher energy state, and then re-emit a new photon when they drop back to the original energy state.

If you had simply typed the question in the subject line into a Google search you would have had your answer.

https://www.physicsforums.com/insights/little-excuse-ask-question-cold/

I don't feel that the OP's question could have been answered with a simple google search. I say this because I was going to post an answer earlier today and then realized that even I didn't know how reflection behaved when it came to single particles. So I searched google. The first few pages turned up little to help me. I then typed up an entire post to answer one of the OP's latter questions before I read some of the responses here and realized I still didn't know what I was talking about. :rolleyes:
 
  • #8
phinds
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... I read some of the responses here and realized I still didn't know what I was talking about.
Yes, but that's normal for you (thanks for the setup :smile:) but I did think that the first Google hit I got pretty well answered the question
 
  • #10
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It's been brought to my attention that I failed to properly commend all of you who have answered my questions. Therefore, I'm correcting my mistake with this comment. Thank you for taking the time and energy to answer my questions.
 
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  • #11
jbriggs444
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It's been brought to my attention that I failed to properly commend all of you who have answered my questions. Therefore, I'm correcting my mistake with this comment. Thank you for taking the time and energy to answer my questions.
It's not so much a simple "thank you", that we are after. It is feedback that shows that you are paying attention and taking our responses on board. And feedback that gives us a clue about where our responses are hitting or missing the mark.
 
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  • #12
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When I was writing this question, I wrote it with a sense of arrogance. The reason for this is because I'm a skeptic about today's model of the atom, so I asked this question with pride rather than curiosity. Another fault is that I did not write this question anticipating to take these answers to heart. So not only was I prideful, but also deceptive. The past is by definition unchangeable, meaning all I can do at this point is ask for all of your forgiveness.
 
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  • #13
davenn
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The reason for this is because I'm a skeptic about today's model of the atom, so I asked this question

OK so as I said in response to you in your other thread

how about starting again
state your age and education … that gives us an idea to what depth of discussion people can respond
that you are able to understand

Clearly state what you do know and understand ( within the realms of todays known physics)
about this particular topic :smile:
 
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  • #14
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First off, I thank you for your generosity in giving me this second chance. Now as to my age and education, I'm 17 and a high school senior. My misunderstanding lies in how electrons absorb photons. I've researched this topic a bit and what I found is that when an electron absorbs a photon, the photon shrinks to the size of the electron when it comes into contact with the electron. There are two problems this raises in my mind. One, this process at least to me, seems to characterize light as silly putty, meaning it has no distinct shape or form. This, however, could not be the case as light is a particle and as far as I know, which is not much, a fundamental part of being a particle is having an unalterable form. My second question is a thought experiment. A photon is one wavelength of whatever beam of energy it is from. This, in turn, means photons will be much larger than electrons. If a photon came into contact with two electrons at once or another electron while it was in the process of being absorbed by a different electron, what would happen? This scenario seems plausible to me because an electron is like the limiting reactant in a chemical reaction. A photon moves and interacts at the speed of light, but that to me does not matter since an electron can only do things at the speed of an electron. This should, in theory, leave a large enough window open for this scenario to happen. And again, thank you for giving me a second chance.
 
  • #15
Dale
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I've researched this topic a bit and what I found is that when an electron absorbs a photon, the photon shrinks to the size of the electron when it comes into contact with the electron.
What source said that? It seems like either you are being misled by a very bad source or you are substantially misunderstanding a good source.

First, a free electron cannot absorb a photon. An atom can absorb a photon, and the atom does so by raising an electron to a higher orbital.

Second, the size of the photon and the size of the atom are irrelevant. What matters is that the energy match. Also, a photon doesn’t really have a size, since size is usually considered to be the distance between subparticles and a photon has no subparticles.

as I know, which is not much, a fundamental part of being a particle is having an unalterable form ... A photon is one wavelength of whatever beam of energy it is from
These are also not correct. Can you identify the source for these impressions?
 
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  • #16
davenn
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First off, I thank you for your generosity in giving me this second chance. Now as to my age and education, I'm 17 and a high school senior. My misunderstanding lies in how electrons absorb photons

And again, thank you for giving me a second chance.

My like of your post was for those two comments, thankyou for that :smile:

Pretty much everything in between those comments, and as Dale commented on, is unfortunately incorrect :frown:

But there is hope :smile: stick around and be prepared to learn some good stuff.
There are some incredibly knowledgeable people on this site ( much better than me) and they will be happy to lead you in the right direction
I know I have learnt a heck of a lit in the last 7 or 8 years that I have been here … particle physics isn't my forte …
I'm into geology, Astronomy and electronics


Dave
 
  • #17
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My source for my information on electrons and photons is Wikipedia. As for the unalterable form, I was the source. Lastly, the source that concerned the size of a photon being one wavelength was, again, Wikipedia. That said, I thank you both for your guidance in this conundrum.
 
  • #18
jbriggs444
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My source for my information on electrons and photons is Wikipedia.
That is not an adequate reference. Give us a URL at least. As @Dale points out, the understanding you took away from that page is incorrect.
As for the unalterable form, I was the source.
A more standard characterization is that an elementary particle has no underlying structure. It is not constructed from other pieces. "Form" and "shape" are not meaningful attributes for an elementary particle.
Lastly, the source that concerned the size of a photon being one wavelength was, again, Wikipedia.
Again, an actual reference would be helpful. What I see at https://en.wikipedia.org/wiki/Photon is that the word "size" is not used anywhere.
 
  • #20
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By specifying "visible light" in the original post, the statement is probably referring to the wave length being too long to "image" a molecule. The wavelength of visible light runs (roughly) from 380nm to 750 nm (3800 to 7500 Angstroms), which is much greater than the dimension of a molecule (a few dozen Angstroms). So the wavelength of visible light is much too long to resolve a molecule. That is different from the absorbtion issue.
 
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  • #21
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I believe the reason why it is not on Wikipedia is that I misunderstood it. I am thankful that you all have corrected my mistake though.
 
  • #22
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Can electrons absorb a photon?
Russian physicist V. Rydnik wrote:
' Now take the electron. Even if its velocity is close to that
of light – 10^10 cm/s – it will have a momentum of only
about 10^-17 g cm/s. The gamma photon used for
illumination has a very short wavelength ( say, 6 10^13 cm)
and a momentum of 10^-14, which is thousands of times that
of the electron. So, when a photon hits an electron, it is like
a railway train smashing into a baby- carriage.’
/ Book: ABC’s of quantum mechanics. By V. Rydnik. Page 98-99. /
---
 
  • #23
looking at the way you are describing photons and eletrons, it sounds like you are assuming that they are some sort of physical round blob like a ball. in quatumphysics they are not realy like that.
 
  • #24
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Yeah, I did have a misunderstanding. As of now though, it is cleared up.
 
  • #25
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Can electrons absorb a photon?
Russian physicist V. Rydnik wrote:
' Now take the electron. Even if its velocity is close to that
of light – 10^10 cm/s – it will have a momentum of only
about 10^-17 g cm/s. The gamma photon used for
illumination has a very short wavelength ( say, 6 10^13 cm)
and a momentum of 10^-14, which is thousands of times that
of the electron. So, when a photon hits an electron, it is like
a railway train smashing into a baby- carriage.’
/ Book: ABC’s of quantum mechanics. By V. Rydnik. Page 98-99. /
---
It seems that he applies classical mechanics to electrons moving with relativistic speeds. Relativistically, the momentum increases without limit as the speed approaches the speed of light.
Besides, what is the point of this? Can photons be absorbed by atoms? How does the momentum of an atom at rest compare with that of a photon?
 
  • #26
phinds
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It seems that he applies classical mechanics to electrons moving with relativistic speeds. Relativistically, the momentum increases without limit as the speed approaches the speed of light.
Besides, what is the point of this? Can photons be absorbed by atoms? How does the momentum of an atom at rest compare with that of a photon?
The person who posted that has left the building
 
  • #27
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...
First, a free electron cannot absorb a photon. An atom can absorb a photon, and the atom does so by raising an electron to a higher orbital.
...

Why, if the atom absorbs the photon and not the electron in the atom, the energy of one photon is not used by the atom to raise more than one electron to a higher orbital and/or even to increase its speed?
 
  • #28
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The formula of '' a free electron'' is: E=h*f (cannot absorb a photon ? ! )
The formula of an electron in atom is : E=-me^4/2h*^2= -13,6eV (can absorb a photon ? ! )
=====
 
  • #29
Dale
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if the atom absorbs the photon and not the electron in the atom, the energy of one photon is not used by the atom to raise more than one electron to a higher orbital
I don’t know. Maybe it can, it is just a low probability.

and/or even to increase its speed?
That certainly does happen commonly. In fact usually the atom’s speed must change in order to conserve energy and momentum.
 
  • #30
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I don’t know. Maybe it can, it is just a low probability.

That certainly does happen commonly. In fact usually the atom’s speed must change in order to conserve energy and momentum.

Ok, but, if the atom can use the energy of one photon for multiple transitions + to change its speed, why it doesn't happen all the time? Why the light is not absorbed completely? Maybe because the electrons in the atoms, not the atoms, are absorbing the photons?

Regarding your statement that a free electron cannot absorb a photon, how about free electrons in metals? If I recall correctly, they can and do absorb photons, regardless of wavelength.
 
  • #31
DrClaude
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Ok, but, if the atom can use the energy of one photon for multiple transitions + to change its speed, why it doesn't happen all the time? Why the light is not absorbed completely?
There are two conservation laws at work: conservation of energy and conservation of momentum. Because of the latter, the change in speed is not arbitrary, so the absorption is still discrete.

Maybe because the electrons in the atoms, not the atoms, are absorbing the photons?
That's a non-sequitur. The fact that light is completely absorbed or not has nothing to do with whether one should think of the electron or the electron+nucleus doing the absorption.

Regarding your statement that a free electron cannot absorb a photon, how about free electrons in metals? If I recall correctly, they can and do absorb photons, regardless of wavelength.
That's different meaning of "free electron." In a metal, the positively-charged nuclei are still there.

Basically, the point is that you need a dipole to absorb a photon. There has to be two opposite charges to couple to the electromagnetic field.

The misunderstandings probably comes from the fact that, since nuclei are much heavier than electrons, most models are based on fixed nuclei, where it looks like the electron is doing the absorption, and many physicists are sloppy in their explanations. But without a positive charge nearby, nothing beyond Compton scattering would happen.
 
  • #32
Dale
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Ok, but, if the atom can use the energy of one photon for multiple transitions + to change its speed, why it doesn't happen all the time?
Again, I don’t know that it can happen. That doesn’t mean that it can and it doesn’t mean that it can’t. I just don’t know.

However, IF it can happen then the reason that it doesn’t happen all the time as I said is that it must have a very low probability.

Maybe because the electrons in the atoms, not the atoms, are absorbing the photons?
No, it is simply impossible for an electron to absorb a photon. An electron cannot absorb a photon and conserve both energy and momentum. I encourage you to sit down and work through the math yourself. Try to figure out a way for an electron to absorb a photon and conserve energy and momentum.
 
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  • #33
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Interesting explanations. Thank you DrClaude!

There are two conservation laws at work: conservation of energy and conservation of momentum. Because of the latter, the change in speed is not arbitrary, so the absorption is still discrete.

That's a non-sequitur. The fact that light is completely absorbed or not has nothing to do with whether one should think of the electron or the electron+nucleus doing the absorption.
...

What conservation law would prohibit a free atom to absorb a photon, any photon, and change its speed accordingly? The absorption is discrete only because the electrons need specific energies in order to raise to a higher orbital. If the energy of one photon could be used both for the transition of the electron (or more electrons) and for a translation of the whole atom (in addition to the one associated with the transition), the absorption of the light would be continuous and possibly complete. That's what I meant. That's why I thought that the electrons are "receiving" the photons and, together with the rest of the atom, absorb them, if the energy is right. This would also explain why only one electron in the atom raise to a higher orbital.
 
  • #34
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... No, it is simply impossible for an electron to absorb a photon. An electron cannot absorb a photon and conserve both energy and momentum. I encourage you to sit down and work through the math yourself. Try to figure out a way for an electron to absorb a photon and conserve energy and momentum.

I wrote electron in the atom (see below) and I explained above what I meant.

Why, if the atom absorbs the photon and not the electron in the atom, the energy of one photon is ...
Thank you for your replies!
 
  • #35
jbriggs444
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I wrote electron in the atom (see below) and I explained above what I meant.
What is the operational difference between an electron [in an atom] absorbing a photon and an atom with an electron absorbing a photon?
 

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