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I sought the answer to the question about amplitude of light waves first, but I was actually thinking about whether the wavelength is the only property of a single quanta of light. I suppose direction may be another, since it can be focused.

Now my question is about wavelength only...do photons actually have a wavelength? Or is that only a property that can be measured based on interactions with other particles (as in, how the photon can affect the state of an electron)?

Qmechanic
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phil v
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    if it does not have any frequency,how can you measure the energy of a single photon?as you can see in plank's formula. on the other hand,think about de Broglie wavelength,and for your second question,search for photoelectric effect. –  Mar 05 '15 at 14:38
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    possible duplicate of How can a Photon have a "frequency"? – kenorb Mar 05 '15 at 14:50
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    What I thought at the time (before reading QED by Feynman) is that light could be a wave - he says no, it comes in particles. So those photons just come straight on? Or do they vibrate as they go? Or are they themselves the vibration (disturbances in the "fabric")? Hopefully that clarifies the question - and its ok to say we don't know yet if that's the answer. Something has to give light its wavelength (that's the energy then?) BTW, I'm no physicist just curious about physics. – phil v Jul 05 '17 at 12:37

2 Answers2

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You are asking about wavelength, but your question is a problem not very well-posed. Then, let's say so: from a single photon we cannot get a conclusion about it characteristics. Only if we know the beam to which the photon belongs, can we know the wavelength (or wavelengths see below).

Next, the wavelength is the result of preparation of the beam, it is not the result of the measuring procedure. In order to measure the wavelength, we perform different experiments, e.g. typically, interference, and for detection, the photons interact ultimately with all sort of materials, e.g. by the photoelectric effect. But, again, it's at preparation that we establish the wavelength.

Now, about the wavelength of a beam, it is rarely well defined, i.e. sharp. See a couple of examples:

1) thermal light, which has a mixture of wavelengths;

2) coherent light, which has a quite low spread in wavelength;

3) plane-waves, which have a well-definite wavelength, but are an idealization - not truly realizable in practice.

So, usually, the beam, and with it the photons in it, practically don't have a well-defined wavelength.

Sofia
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    I don't think this really answers his question... – Sean Mar 05 '15 at 14:45
  • @Sean and why don't you think that this really answers the question? Can you be some more clear? Isn't the question about wavelength? And isn't the wavelength or the wavelengths, according to how we prepared the wave-packet? – Sofia Mar 05 '15 at 14:52
  • @Sean at a 2nd look, I reorganized my answer. I think the answer was O.K. but not properly organized. Though, it you think differently, can you tell me clearly why? – Sofia Mar 05 '15 at 15:03
  • He asks if a single photon can have a frequency. You give all these examples of beams which have a variety of frequencies, and then take an agnostic "we can't know which frequency a single photon" possesses. That's misleading. Otherwise, how could you talk about the energy of a single photon? – Sean Mar 05 '15 at 15:04
  • @Sean please look at the question: "I was actually thinking about whether the wavelength is the only property of a single quanta of light." And next, "do photons actually have a wavelength? Or is that only a property that can be measured based on interactions with other particles ". And my answer is: "no, wavelength characterize the wave-packet", and "it is determined at preparation, not by measurement". One prepares a wave-packet with certain wavelength, measurement isn't preparation. Maybe you're influenced by previous problems, but, this one is different, *by how it is written*. – Sofia Mar 05 '15 at 15:11
  • @Sean I repeat, by measuring a single photon you learn *nothing*, you have to know the beam to which it belongs. Recall the collapse. If you get at the measurement some energy, that doesn't mean that it was the energy before measurement. – Sofia Mar 05 '15 at 15:22
  • My answer is opposite of Sofia's but oddly, I don't think either of us are wrong. I think Sofia is just a bit further down the rabbit hole than I am. – Jiminion Mar 05 '15 at 16:24
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Yes, a photon is defined by its wavelength, which also directly relates to its energy.

I'm not sure what you mean by interacting with other particles, but a simple glass prism is able to separate the different wavelengths of white light into its constituent bands.

The wavelength of a photon of light is related to its energy by planck's constant.

http://en.wikipedia.org/wiki/Planck_constant

Jiminion
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  • no, I am sorry, a single photon has no defined wave-length. What is a single photon? A plane wave, spread over all the space? Only then it may have a well-defined wavelength. – Sofia Mar 05 '15 at 16:39
  • Does a single photon have energy? – Jiminion Mar 05 '15 at 16:44
  • The same problem as with wavelength. But, why you ask, it's strange, you know that from a single particle we cannot guess the wave-function. You know that we can measure a single particle and get a result that won't tell us anything precise, and only many measurements on identical particles would indicate what is the wave-packet. And you know that in a wave-packet there are many values of $k$. So, the photon doesn't have a precise $k$. Tell me the wave-packet and I'll tell you the expectation value of $k$, or of the energy, etc. *This* is correct, a single measurement tells *nothing*. – Sofia Mar 05 '15 at 16:53
  • The OP is not asking that. The OP is asking "do photons actually have a wavelength?" How could the answer to that be 'no' if the Planck relationship is valid? – Jiminion Mar 05 '15 at 17:09
  • Look please as the exact question "I was actually thinking about whether the wavelength is the only property of a *single quanta of light." Did Planck deal with a single quanta*? Where did you see that? – Sofia Mar 05 '15 at 17:15
  • Yes, look at the question: "Now my question is about wavelength only...do photons actually have a wavelength?" – Jiminion Mar 05 '15 at 17:16
  • but, if you have doubts about what I say, you do believe in the Curious Mind, don't you? Then, please get into the hbar room, and see what he said. – Sofia Mar 05 '15 at 17:17
  • I answered both about 1 photon, and about *photons*. What more than that can I do? – Sofia Mar 05 '15 at 17:19
  • @Jiminiom - I mean interacting with an electron. My understanding as a laymen is that a photon of light is emitted by a source and can interact with electrons (be absorbed and emitted by electrons). I can imagine a single photon moving through space-time, but does it go straight or with a specific vibration? If with vibration, then is it simple or complex? With a laser (focused single frequency) it seems that photons move in harmony...but is it the photons that vibrate at f or are they emitted at f? Which gives the light its wavelength? – phil v Jul 05 '17 at 12:46