I am really confused about the part where the photon shares a part of its energy with the electron. It is said that the photon loses some of its energy to the electron causing the change in its wavelength. In my understanding photon isn't something that contains energy, it's the energy itself. A single photon has a fixed amount of energy (E = hv) ;the smallest possible value of energy of the photon at that particular frequency. How can it break into something smaller, doesn't that voilate the postulates of Plank and Einstein about the quantum nature of light? Or does a photon break into many, in that case, is the quantized nature of light even meaningful anymore?

Moreover if the x-ray photon has surplus energy after knocking the electron out of the atom, why doesn't it contribute to the electron's kinetic energy? Like it does in the case of photoelectric effect. And it leads to another confusion I have now. If photons can share part of their energy then can't we simply increase the number of photons hitting the photoelectrode such that the free electrons can slowly store enough energy to break the required energy thershold.

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    $\begingroup$ "In my understanding photon isn't something that contains energy, it's the energy itself." What does this mean? No, really, what do you think this sentence actually means in practical terms? $\endgroup$
    – ACuriousMind
    Nov 30, 2023 at 18:36
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    $\begingroup$ the smallest possible value of energy of the photon at that particular frequency But the frequency can change after the interaction. $\endgroup$
    – Ghoster
    Nov 30, 2023 at 18:48
  • $\begingroup$ well, I'll try to explain. The thing with a photon is that the amount of energy it has is the practical lower limit of the energy light (photon) can have at that particular frequency. The amount of energy can't get any lower (excepting 0), for if it does, would the photon really be a quantum of energy? For one photon we can have only hv amount of energy nothing more nothing less. If after the interaction we have a photon with a different frequency, it should be a different photon emitted by the electron, right? $\endgroup$ Nov 30, 2023 at 18:58
  • $\begingroup$ The statement: energy is quantized (or appears as packets of quanta hv) is made for systems whose energy spectrum is discrete...like standing waves in a closed box will have energy as integer multiple of a fundamental frequency. But this frequency depends on parameters of the box. Once this photon escapes the box, it can interact with other systems and change its frequency arbitrarily. In this case the idea of fundamental frequency doesn't apply $\endgroup$
    – paul230_x
    Nov 30, 2023 at 19:01
  • $\begingroup$ Alright, that makes some sense. But what about the photoelectric effect part I mentioned? The photons in that case are out of the box, so nothing is stopping them from knocking free electrons while having a frequency that is lower than the critical frequency. $\endgroup$ Nov 30, 2023 at 19:07

1 Answer 1


Don't be mislead by the wiki explanation of the Compton effect. The full x-ray photon can/could be absorbed by one electron .... the electron can change energies in may ways leading to other photons etc etc . Yes the first photon can be thought of as pure energy .... and like all photons it must resonate with an electron (probably just one). Many over zealous scientists would love to break the photon ... and the Compton affect gives then the excuse to do so ... but the collision is very high energy and nobody knows the inner complexities.

  • $\begingroup$ So does Compton effect of does it not reinforce the quantum nature of light? Given that nobody knows what actually is going on. $\endgroup$ Dec 1, 2023 at 3:20
  • $\begingroup$ from wiki .... this is where he proves the quanta nature : "The energy of light quanta depends only on the frequency of the light. In his paper, Compton derived the mathematical relationship between the shift in wavelength and the scattering angle of the X-rays by assuming that each scattered X-ray photon interacted with only one electron" ..... only one electron. $\endgroup$ Dec 1, 2023 at 15:15

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