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A white light, as we all know, is composed of seven lights VIBGYOR. Each of the component lights has distinct frequency ranging from one value to another. So, when the photons have wavelength of 600 nm it becomes a yellow light and when it changes to 700 nm it becomes a red light. So, does this imply that the frequency with which a group of photons vibrate determine the colour of light? Well, in that case, with what frequencies are photons vibrating for a white light?

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  • $\begingroup$ Actually, no, we do not know that. The definition of "white" is quite different, and depends on whether you're talking about photopic (human eye) response or uniform frequency densities analogous to "white noise" in acoustics. $\endgroup$ – Carl Witthoft Jun 20 '14 at 11:27
  • $\begingroup$ Almost duplicate; at least highly relevant:physics.stackexchange.com/q/78933/26076 $\endgroup$ – Selene Routley Jun 20 '14 at 14:23
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To get a perception of a white light, one has to observe simultaneously many photons having different frequencies and obeying a certain distribution function.

Keep in mind that a color is not a physical property, since it is resulted from human brain's interpretations of signals coming from eyes receptors.

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  • $\begingroup$ Well, in that case, when a photon is simultaneous cycle of vibrations in different frequencies, doesn't that give rise to certain cases like induced action just as in Laser action ? $\endgroup$ – Avery Jun 20 '14 at 8:27
  • $\begingroup$ I did not get what you have said $\endgroup$ – freude Jun 20 '14 at 8:29
  • $\begingroup$ What I meant to say was that if several photons are under simultaneous cycles of oscillation, wouldn't that result in generation of induced actions just like that in production of laser? Won't that creat a new form of electromagnetic radiation? $\endgroup$ – Avery Jun 20 '14 at 8:37
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Light that our eyes, or other system of detectors, would perceive as "white" could have several different maekups:

  1. It could be a stream of photons all in different energy / momentum eigenstates. That is, there could be a population of photons, some of which are red wavelength, some blue, some green and so forth as in freude's answer; OR

  2. It could be made up of photons all in exactly the same quantum state. You really can have one, lone, sole "white" photon all on its own. A photon, like any other quantum object, can be in a pure quantum superposition of basis eigenstates (eigenstates of whatever observable you care to work with). There is no reason why a photon has to be in a pure energy / momentum eigenstate. Its state could be such that there is a complex amplitude $\psi(\nu)$ to be observed at wavelength $\nu$. For more information about this idea, see my answer to the following question How can we interpret polarization and frequency when we are dealing with one single photon?

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  • $\begingroup$ Huh? A single photon has a defined wavelength. How will that be white? Alternatively- any photon absorbed by a rod rather than a cone will be interpreted as "white" because rods are frequency-insensitive. $\endgroup$ – Carl Witthoft Jun 20 '14 at 11:30
  • $\begingroup$ @CarlWitthoft No it doesn't: a photon with a single wavelength is in an energy eigenstate. It doesn't have to be so, just as an electron doesn't have to have a well defined position (i.e. an electron doesn't have to be in a position eigenstate). Have a look at my other answer. If a photon were prepared so that it was in a superposition of energy eigenstates, and it coupled with a whole mass of red / green/ blue detectors (or something like this), it would have a certain probability to be detected by each. You really could do such an experiment one photon at a time ... $\endgroup$ – Selene Routley Jun 20 '14 at 11:34
  • $\begingroup$ ... and tally up the number of absorptions by each colour detector (cone, if you like, but hopefully you can see that the idea is much more general), and you would conclude that each photon on its own had a spectral spread. Such perfectly coherent white light, i.e. comprising photons all in the same pure quantum state, is indeed possible. It would be pretty contrived and weird, though, and freude's answer is the description of "everyday" white light. $\endgroup$ – Selene Routley Jun 20 '14 at 11:38

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