How many photons are reflecting?

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Discussion Overview

The discussion revolves around the question of how many photons from a diode laser are reflecting off a mirror. Participants explore the relationship between the laser's power, the energy of individual photons, and the reflectivity of the mirror, while also delving into the nature of photons and their properties.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that the number of photons reflecting depends on the mirror's reflectivity and the intensity of the laser.
  • One participant calculates that a 10 mW laser emits approximately 3.3E16 photons per second, but emphasizes the need for the mirror's reflectivity to determine how many are reflected.
  • Another participant questions how many photons are "touching" the mirror at any given moment, suggesting that this is a statistical question and may not have a definitive answer.
  • Some participants argue that photons do not have a measurable length, while others discuss the implications of defining a photon's length in terms of its wavelength.
  • There is a debate about whether it is more accurate to say that photons do not have a length or to assert that their length is zero.
  • One participant expresses frustration with the original question, suggesting it lacks meaning and clarity.
  • The discussion touches on the nature of electron emission and whether an electron must move at the speed of light to emit a photon, with some participants clarifying misconceptions around this topic.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the original question regarding the number of photons reflecting off the mirror. There are multiple competing views on the nature of photons, their properties, and the implications of their behavior in this context.

Contextual Notes

Participants express uncertainty about how to quantify the number of photons "touching" the mirror at a specific moment, highlighting the statistical nature of photon behavior and the complexities involved in defining photon properties.

  • #31
Simon Bridge said:
but I'm guessing that English is not your first language. Is that the case?

Yes third only, sorry for language mistakes.

Maybe someone may estimate what density of light is requared so that notable amount of cases atoms would absorb two photons simultaneously (instead of 1).

Lets say 20% of cases we can consider like notable.

This also would give me some info I was looking by asking indirect question like one at the beginning of this tread.

So how strong light could be to make photoeffect with photons a bit weaker (longer wave lenght) then usually needed.
 
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  • #32
Please do a search on multiphoton photoemission. This is a very well-known phenomenon that we have already been using. The original photoelectric effect is a single-photon photoemission phenomenon!

This thread has meandered all over the place.

Zz.
 
  • #33
Alvydas said:
Why? You may spread light onto very big photosensitive equipment and count them individually. Or decrease number of photons by semitransparent mirrors and count them individually also.
I don't know if you are aware of the uncertainty principle in quantum mechanics. For coherent states, like a laser, there is an uncertainty relationship between the phase and the number of photons.

http://en.wikipedia.org/wiki/Coherent_states
 
  • #34
Alvydas said:
If you think photon is a point than maybe question can be transformed to:
what size is space where we may find it at current moment?
I do not think that a photon is a point, but I briefly discussed the approach you are suggesting here back in post 8.
 
  • #35
Yes third only, sorry for language mistakes.
No worries: we are all very skilled communicators so we can compensate as I'm sure you do when someone tries to talk to you in your first language when they are not used to it... well most of us anyway. Your skills are better than a lot of native English speakers I've met ;)

This also would give me some info I was looking by asking indirect question like one at the beginning of this thread.
It is better to be direct - especially when you are talking to scientists. You will get much better help a lot sooner. Don't worry about appearing silly or anything - we've all been there: we understand.

ZapperZ has given you a good lead to follow. Enjoy :)
 
  • #36
Alvydas said:
But if photon is a point it can not overlap each other and therefore always acts alone on an atom.
First, as I said above, it is bad to think of a photon as a classical point particle, which appears to be what you are describing. However, because an atom does have size even if photons do not you could still get multiple photons interacting with the same atom at a given time.
 
  • #37
Furthermore, the multiphoton effect has nothing to do with the "size" of the photon, but rather the time scale of the interactions. In multiphoton photoemission, it is the time for an excited state to decay back to its ground state. If a photon can excite a particular electron while it is in an excited (but bound state) of the solid before it decays back to the valence band, then it has a chance to be excited to an even higher state. The factor that affects this includes the photon density per unit area, and the decay rate of the excited state. It has nothing to do with the size of the photon.

Zz.
 
  • #38
So I can send strong infrared beam to a mirror and get reflected visible light ?
But not due surface heating, just because of reemitting photons with double energy by the mirror.
Am I right?
If so what (approximately) power of incoming laser light I need to have at 1 square mm of mirror's surface? (to be effect visible)
 
  • #39
You are referring to Laser Up-converting, which is a process that can be used successfully. Google it - there are a lot of links. Most lasers work with a single 'pumping' wavelength applied and a longer wavelength laser light is produced as atoms return to a lower state. However, it is possible to pump some lasers with more than one wavelength and the resulting EM will have a higher energy than either of the pumping sources because the atoms are pumped to a level equal to the sum of the energies.

This will only happen under rare circumstances where the probability of an atom that has already been 'pumped' once, will decay is less than the probability that another incoming photon will pump it further. In most circumstances, the once-excited atom will decay before this can happen.
 
  • #40
Alvydas said:
So I can send strong infrared beam to a mirror and get reflected visible light ?

That has not been shows to can occur. You are applying different physics to different phenomenology. The multiphoton effect is on a photoemission phenomenon, NOT a "reflection".

There's just something not quite right with what you are thinking and with your question.

Zz.
 
  • #41
ZapperZ said:
That has not been shows to can occur. You are applying different physics to different phenomenology. The multiphoton effect is on a photoemission phenomenon, NOT a "reflection".

There's just something not quite right with what you are thinking and with your question.

Zz.

Yes, I agree it's a bit tenuous but, there again, when light is reflected in a mirror, the interaction isn't just one photo - one atom, in any case, but with the bulk material. Perhaps that should have been emphasised more earlier on.
I thought this up-conversion laser effect occurred in the solid state so the effect would be not 'specular' reflection but would still involve light coming off a surface when illuminated with two other, longer, wavelengths.
 
  • #42
Alvydas said:
So I can send strong infrared beam to a mirror and get reflected visible light ?
But not due surface heating, just because of reemitting photons with double energy by the mirror.
Am I right?
Not to my knowledge, no. Reflection has little, if anything, to do with atomic emission.
 
  • #43
sophiecentaur said:
Yes, I agree it's a bit tenuous but, there again, when light is reflected in a mirror, the interaction isn't just one photo - one atom, in any case, but with the bulk material. Perhaps that should have been emphasised more earlier on.
I thought this up-conversion laser effect occurred in the solid state so the effect would be not 'specular' reflection but would still involve light coming off a surface when illuminated with two other, longer, wavelengths.

I didn't want to get into that because the OP has a lot more issues with other basic understanding.

It is my opinion that this thread is nothing but a jumble of disconnected information that the OP has no ability to process correctly. This is one of those times where I often wish that people would ask just simple, basic question first before attempting to find something beyond what they are capable of understanding.

Zz.
 
  • #44
Yep.
I agree entirely. I think people are reluctant to ask the most basic questions for fear of appearing ignorant.
 
  • #45
Thank you all.
I have got some useful info.
I will try to adjust my style of asking to be more clear.
 

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