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Photons
Feb 17, 2011 16:17:07 GMT 1
Post by carnyx on Feb 17, 2011 16:17:07 GMT 1
We hear that the double slit effect can be carried out with photons.
So, how many photons in a wavelength of say a monochromatic green light? And does the number of photons change with amplitude?
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Photons
Feb 17, 2011 17:03:25 GMT 1
Post by speakertoanimals on Feb 17, 2011 17:03:25 GMT 1
There is no MEANING to the question how many photons IN a wavelength. If I have a beam of light of one fixed frequency shining at me, the question as to HOW MANY photons I receive per second (or per wavelength if you want to factor that in) depends upon the intensity of the beam. The more intense, the more photons per second. But I can still have a very, very WEAK beam (one photon per fortnight), the photon and the asociated wavefunction can have a definite wavelength, but the question as to how many photons per wavelength is till meaningless, because the whole point about the wavefunction is that the photons DON'T have any definite position until I detect them. So, the number per wavelength is either meaningless, or if you insist on trying to assign a meaniong to it, any number you like depending on how intense or weak the beam is. The problem is you are using classical notions and trying to apply them to something which is NOT classical by definition. Like asking what gravity tastes like, it is meaningless. I think what yopu're trying to get at is that it can be carried out with SINGLE photons. So, what you do is have a detector that can detect SINGLE photons. You turn the intensity of the light source right down (better have it in a dark room or properly shielded), so that you can then compute the probable number of photons in the apparatus at any one time, based on how often, on average, they are counted on the screen. So, if they arrive at a rate of one per second, then the average separation between photons is one light-second, which is still quite a large distance, so the chances of there being more than one at a time inside your meter-long apparartus is fairly small. In fact, you could probably turn up the intensity a bit, because counting only one per second it is going to take a while to discern the pattern! See here for an actual set-up: www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/SinglePhotonInterference/SinglePhotonInterference.html
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Photons
Feb 17, 2011 20:04:27 GMT 1
Post by carnyx on Feb 17, 2011 20:04:27 GMT 1
@sta
As it is, I understand a photon as a discrete packet of energy that has a specific value, and so must be equivalent to an EM wave-train at a frequency, amplitude and number of cycles, that would contain the same amount of energy.
For example, if photons travel at the speed of light, do they have a length?
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Photons
Feb 17, 2011 20:54:58 GMT 1
Post by speakertoanimals on Feb 17, 2011 20:54:58 GMT 1
Nope -- you've asked this before, as I recall.
You've got the thing the wrong way round. A stream of photons is equivalent to (when you have enough of them) to some em wave. But that doesn't mean that a single photon is.
you are missing the fact that a classical em wave carries energy continuously, hence you can think of how much energy per second is transmitted, hence how much per wavelength, if you like. Except the energy trasferred by a photon ISN'T continuous, you get 1 quantum of energy every time a photon turns up, and none in between.
Hence if you have enough photons, so many turn up every instant that you no longer notice the discrete nature of the transfer, and you can approximate it very well by a continuous wave model. but when you are down at the single photon level, you can't do that.
As you've done so often before, you keep trying to analyse the quantum situation using classical concepts, and get congused when thety no longer fit. No surprise, we already KNOW they don't, which is why quantum theory was invented after all......................
but then you'd have to understand classical wave theory before you could get to where it breaks down, and as your daft claims about density and frequency show, you don't even get the basics of that, hence trying to get a handle on anything else is pointless.
If? Whatb other speed do you think they travel at? (refractive index aside for the moment).
Wavelength ISN'T length of photon, get over it!
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Photons
Feb 17, 2011 21:06:36 GMT 1
Post by abacus9900 on Feb 17, 2011 21:06:36 GMT 1
The standard rule of thumb is that when a wave is very much larger than the scale you are probing, then you are going to see wave behaviour in your EM wave.
When the wave is very much smaller than the scale you are using then you are going to see particle-like behaviour.
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Photons
Feb 17, 2011 21:35:07 GMT 1
Post by speakertoanimals on Feb 17, 2011 21:35:07 GMT 1
Except you shouldn't mix up spatial and temporal scale!
So, when doing optics, you can get decent results using wave theory as long as you aren't looking on small scales AND as long as your intensity is large enough that the scatter-gun arrival of photons won't be noticed.
If you have such a low intensity that you only have one photon in you set-up at a time, then you stil have quantum effects (pattern builds up single photon by single photon), even if what you are looking at is something boring and classical, like a scene viewed through a lens.
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Photons
Feb 17, 2011 21:36:04 GMT 1
Post by carnyx on Feb 17, 2011 21:36:04 GMT 1
STA,
You are closing in on the question!
At Last! Hooray!
A stream of photons can be approximated very well by a contiuous wave model.
Now, what is that approximation? In terms of frequency/ amplitude vs number of photons?
(And so, we can get a handle on the point of departure between the two usages that Abacus mentions)
So, STA, could you oblige with the answer?
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Photons
Feb 17, 2011 21:46:17 GMT 1
Post by speakertoanimals on Feb 17, 2011 21:46:17 GMT 1
If you knew that already WHY did you ask?
It depends on what you are doing, whether the behaviour is strictly quantum, or approximately classical. There is no one SIMPLE answer.
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Photons
Feb 17, 2011 21:47:40 GMT 1
Post by abacus9900 on Feb 17, 2011 21:47:40 GMT 1
Except you shouldn't mix up spatial and temporal scale! So, when doing optics, you can get decent results using wave theory as long as you aren't looking on small scales AND as long as your intensity is large enough that the scatter-gun arrival of photons won't be noticed. If you have such a low intensity that you only have one photon in you set-up at a time, then you stil have quantum effects (pattern builds up single photon by single photon), even if what you are looking at is something boring and classical, like a scene viewed through a lens. For Christ sake woman you are not talking to a class! Try doing some explaining for a change, we're not bloody physicists.
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Photons
Feb 17, 2011 21:54:59 GMT 1
Post by speakertoanimals on Feb 17, 2011 21:54:59 GMT 1
And so we descend into the usual nonsense..........................
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Photons
Feb 17, 2011 21:56:50 GMT 1
Post by abacus9900 on Feb 17, 2011 21:56:50 GMT 1
Well? I'm still waiting for an explanation!
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Photons
Feb 17, 2011 22:00:19 GMT 1
Post by speakertoanimals on Feb 17, 2011 22:00:19 GMT 1
I already told you, dimwit, there is no one SIMPLE explanation for the question you are pretending to want answered, which is why that is all you are getting.
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Photons
Feb 17, 2011 22:06:41 GMT 1
Post by carnyx on Feb 17, 2011 22:06:41 GMT 1
Abacus,
I think we have tapped into a well of ignorance.
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Photons
Feb 17, 2011 22:10:06 GMT 1
Post by abacus9900 on Feb 17, 2011 22:10:06 GMT 1
Nuttier and nuttier. It's not that I am a dimwit, it is a matter of you possessing near zero communication skills. Unfortunately this is an insight you do not seem ever capable of experiencing. How on earth your partner puts up with you is hard to imagine. And now I suppose you'll go away and cry in the corner, won't you?
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Photons
Feb 17, 2011 22:11:24 GMT 1
Post by abacus9900 on Feb 17, 2011 22:11:24 GMT 1
Abacus, I think we have tapped into a well of ignorance. Yeah, I have given up any hope of learning anything here. The only thing I have learnt is how barmy STA is.
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