# How many photons are reflecting?

1. Aug 15, 2012

### Alvydas

2. Aug 15, 2012

### Simon Bridge

Depends on the mirror, and the intensity of the laser.

3. Aug 15, 2012

### Staff: Mentor

The energy of a 650 nm photon is E = hc/λ = 3E-19 J. So a 10 mW output would correspond to 3.3E16 photons/s from the laser. Then you would need to know what the reflectivity of the mirror is to determine how many are absorbed vs. reflected.

4. Aug 15, 2012

### Alvydas

You have calculated how many photons was emitted per second.
But I am asking how many of them are touching mirror at current moment.

I think we need length of photon also.

Lest say reflectivity is 100%.

5. Aug 15, 2012

### Staff: Mentor

We don't know. We can't know. It is statistical in nature. And this ignores the difficulties of saying a photon "touches" the mirror in the first place. At the quantum level things are not like they are at our scale.

6. Aug 15, 2012

### Staff: Mentor

I don't think that photons have a length. I cannot even think of a way that the length of a photon could be measured in principle.

As far as the expectation for how many are touching "now", the answer would be zero. This should be obvious from the well known properties of statistical distributions. If you want to know the expectation for how many are touching over some finite time period of duration Δt then simply take 10 mW times Δt to get the energy and divide that by the energy per photon above.

Last edited: Aug 15, 2012
7. Aug 15, 2012

### Staff: Mentor

Well, I think many people think of it as the wavelength of the EM wave. However this really depends on what you view photons as. Personally I only use photons to describe the quantized interaction of the wave, in which case the wavelength of a photon doesn't make any sense. However that is just my own way of thinking of it, mostly as a result of multiple discussions here on PF about what a photon is.

8. Aug 15, 2012

### Staff: Mentor

Yes, I agree. Roughly speaking I think of a photon as a quantized excitation of the EM wavefunction. I guess you could define the photon's length by taking its wavefunction and doing some sort of 95% confidence interval, but that seems awfully artificial.

Last edited: Aug 15, 2012
9. Aug 15, 2012

### sophiecentaur

Yes: if they had "a length" and, of course, all photons of the same energy would have to be identical, then they would all have to have the same "length", whatever mechanism happened to produce them. The only possible value of length that would satisfy this requirement is Zero. This agrees with the accepted 'extent' of a photon.

Consider this, also. The wavelength of visible light is conveniently small to allow you to consider the photons as having a nice short length (in an arm-waving kind of a way and something related to the wavelength) but how long would the photons of an lf radio wave need to be - with a wavelength of a couple of km??? Best not try to allocate a length, I think.

10. Aug 15, 2012

### Staff: Mentor

I never like asserting that the length or size of a particle is zero since that always unfortunately conjures up the idea of classical point particles zipping around, and even non-classically the wavefunction is not a dirac delta in general. So I would rather say that it doesn't have a length than to say that it has a length and that length is zero.

What is your opinion of saying that it doesn't have a length rather than saying that its length is 0?

11. Aug 15, 2012

### ZapperZ

Staff Emeritus
You people should not argue with each other on this matter and let the OP get away with asking a rather meaningless question. I mean, ".... Touching mirror at current moment...."? No one questioned that?

Zz.

12. Aug 15, 2012

### Alvydas

But to emit instantaneously electron could move much quicker then c ?

13. Aug 15, 2012

### Staff: Mentor

Huh? No, an electron has mass, so it cannot move quicker than c. That said, I have no idea how you think that fits in with the rest of everything.

Btw, coherent states, like lasers, don't have a definite number of photons.

14. Aug 15, 2012

### Alvydas

So very short time after emission we have the same atom like before,
because any parts of it can not change their dislocation due c limit so quickly
+ just emitted new photon?

15. Aug 15, 2012

### Staff: Mentor

What does "dislocation due c limit" mean or have to do with emitting a photon. Are you under some confusion that a massive particle must move at c in order to emit a photon? If so, that is not correct, I don't know why you would think that.

16. Aug 15, 2012

### Bobbywhy

Jan27-12 08:23 AM

Alvydas, Have you read the Rules of this forum, as I suggested to you recently? If you had, or if you do, and you have any understanding of the English language, then you will NOT post this speculation here. Remember what happened 10 days ago to this same ficticious whacko off-the-wall stuff. Your thread was locked until you reference some peer-reviesed journal/source. You have only rewittten the same old broken-down false ideas from you last attempt at posting here. Not to mention your ridiculous comparisons of theoretical subatomiic particles such as "fruits" and "apples"! You are either really stupid, or unbalanced, or drunk, or so emotionally attached to these uproven phony wastebasket garbage theories that you cannot control your own actions. Bye bye.

17. Aug 15, 2012

### Alvydas

I do not think that electron must have some proper velocity to emit in classical way.
But instantaneous emission makes some confusion.
Like I mentioned after short time after emission we have the same atom + a new photon.
I am right?
If so (this atom is still is the same) why it can not emit 10 the same photons more?

18. Aug 15, 2012

### Alvydas

I nothing had prosed at this tread. Do you ban even questions?

19. Aug 15, 2012

### sophiecentaur

Either would be fine with me. No value or a value of zero would both avoid a specific length. Excluding the possibility of a value could be hard for some people to accept, though. But the whole idea of a wave function 'collapsing' when a photon is detected implies instantaneous transfer of information etc. etc. which is also hard to accept. It's all a bit too much for a normal brain, I think.

20. Aug 15, 2012

### Staff: Mentor

Hold on. If the emission of a photon is by an electron dropping energy levels in an atom or molecule, the electron does NOT move anywhere. to my knowledge the change in energy levels can be instant because there is no motion here. It can't emit 10 photons because the change in energy levels gives up energy in the form of the photon and the electron is now in a new state. It can't go back and drop energy levels again, we would have to excite it first, which requires energy.