Can electrons absorb a photon?

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Main Question or Discussion Point

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?
 
  • #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
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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.
 
  • #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.
 
  • #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.
 
  • #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.
 
  • #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
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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?
 
  • #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.
 
  • #19
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  • #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?
 

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