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Why does a white photon create different energy photons in the prism. I have learnt that energy of the light is same after entering the prism. Then why does it break into seven colours photons who's energies are different. And how does it maintain the energy conservation. Does the energies of seven photons sum equalize the total energy of white photon?

  • Are you sure there is a white photon? – M. Enns Mar 29 '18 at 00:12
  • @M.Enns white light should be made up by white photon. But I don't know surely. Please help! –  Mar 29 '18 at 00:15
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    White light is a mix of different colors. The prism refracts different colors different amounts and separates them. – M. Enns Mar 29 '18 at 00:17
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    There is no white photon. Nor a black one in case you wonder. A photon has a single frequency, that defined it's color. For white it is a mix of photons which if roughly equally distributed between the different colors gives you white. If you take Zed's prism below and use the outgoing light, and with mirrors bend each color so they come together again it will look white. Many photons – Bob Bee Mar 29 '18 at 01:04
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    "A photon has a single frequency" - is this true? From what I've read here, a photon is an eigenstate of the photon number operator with eigenvalue 1. Taking a look at the linked FAQ, find: 'Thus a general photon is a superposition of monochromatic waves with arbitrary polarizations, frequencies and directions." – Alfred Centauri Mar 29 '18 at 01:55
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    I took a quick look at that FAQ, and I don't understand it. What I can tell you is that in every presentation of QED that I have seen, photon states are by definition single frequency. There may be another way to look at things that I've not seen, but it seems the single frequency picture is certainly the most common. – garyp Mar 29 '18 at 02:41
  • White is not a color, t is a mix of colors. See this video of a rotating disc with colored slices https://m.youtube.com/watch?v=OiUjqWLIWUs – HolgerFiedler Mar 29 '18 at 08:44
  • @AlfredCentauri do you have any other answer for the question? Then please post that. –  Mar 29 '18 at 10:21
  • @garyp, see also the answer here: "For photons, most often we use momentum eigenstates, which are plane waves. Any one photon state of the quantized electomagnetic field can be a superposition of these plane wave states. Notice that plane waves can have different frequencies. So a pure, one-photon quantum state is often in a superposition of energies": – Alfred Centauri Mar 29 '18 at 12:06
  • @AlfredCentauri I saw that. But I associate "photon" with the particle-like behavior where an EM excitation transfers energy and momentum discretely, e.g. exciting a charge carrier in a CCD. The usual Hamiltonian destroys one "photon" while creating an excitation in, for example the CCD material. The description uses harmonic oscillator raising and lowering operators. The Hamiltonian for the EM modes that leads to the raising and lowering operators that, being harmonic oscillators, are applicable to single-frequency modes. (continued ...) – garyp Mar 29 '18 at 17:12
  • (... continuing) @AlfredCentauri But you can create a photon that is a superposition of momentum states as long as all the momentum states have the same frequency. This is how cavity modes are developed. These are single-frequency solutions to the Helmholtz equation. In this picture, the one I'm familiar with, I don't see a place for photons having a distribution of frequencies. But I'm open to learn something. – garyp Mar 29 '18 at 17:15
  • @garyp, here the authors describe a single photon in a superposition of energy eigenstates. From a link at the above link: "Single particles of light can be prepared in a quantum superposition of two different colors, an achievement that could prove useful for quantum information processing." – Alfred Centauri Mar 30 '18 at 12:08
  • @AlfredCentauri Very interesting; I'll have to look into it more deeply to understand the connection to the conventional description of a photon. Also, it's very new, and very difficult to achieve and verify, so it's not something that would apply to this question. – garyp Mar 30 '18 at 19:30

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Each photon is identifyed by its frequency, and there is no frequency perceived as white. A group of photons can be white, not one. Each photon (of different frequency) goes through the prism with a different trajectory but (assuming the prism does not absorb light) the same number of photons that come in, come out.

oyster
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  • "Each photon is identifyed by its frequency" - "According to quantum physics, monochromatic light of frequency , such as the light emitted by a laser, is composed of photons of energy E=h, where h is the Planck constant. Polychromatic light, such as the light emitted by the Sun, contains photons of many different frequencies. However, each individual photon usually has a well-defined frequency and energy. Interestingly, the superposition principle of quantum physics allows for yet another version of polychromatic light: ..." – Alfred Centauri Mar 30 '18 at 12:20
  • "... a single photon in a superposition of two discrete frequencies $_A$ and $_B$. In this case, neither the frequency nor the energy of the photon is well defined. In some sense, such a “bichromatic” photon can be thought of as having two different colors at the same time, only one of which would be revealed if the photon were measured by a spectrometer or detected by eye." - Viewpoint: Photon Qubit is Made of Two Colors – Alfred Centauri Mar 30 '18 at 12:20