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B Which component of light is a photon?

  1. May 12, 2016 #1
    Light is an electromagnetic radiation with two wave components that are perpendicular. Between these two perpendicular waves, where is the photon in this system.
  2. jcsd
  3. May 12, 2016 #2


    Staff: Mentor

    There is no separate photon and wave component.

    Light can behave like a wave or it can behave like a particle depending on the circumstances being measured.

  4. May 15, 2016 #3
    Okay thanks. It behaves like! What is its real nature.
  5. May 15, 2016 #4


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    Staff: Mentor

    When I strike a block of stone with a heavy steel hammer, there are sparks. Between the hammer and the stone, where is the spark in this system?
    "Real nature" is not a meaningful concept in science. If a theory explains and accurately predicts the behavior of a system, we go with that theory until and unless we find something better.

    The best theory we have for explaining and predicting the behavior of light is classical electrodynamics, discovered by Maxwell in 1861. The explanation is that light is the electromagnetic waves predicted by Maxwell's equations, and it pretty much always works.

    However, Maxwell's equations do not completely describe the way that electromagnetic fields exchange energy and momentum with charged particles. It turns out that these interactions always transfer energy and momentum in discrete steps, and we call these steps "photons". That doesn't mean that photons are a "component" of light, or that light is "made up" of photons, or that the photon is even there except when the light is interacting with matter.
    Last edited: May 15, 2016
  6. May 15, 2016 #5
    What does that mean, that the photon is just a probability wave until it interacts with something?
  7. May 15, 2016 #6
    Not a 'probability wave', just fluctuations (waves) in an electromagnetic field.
    'Interacting with something' generally means that the something has absorbed some energy of the wave.
    This results in some electrons gaining energy, however electrons can only transition to a more excited state in discrete steps, (quanta).
    The number of such steps occurring is countable by an appropriate measuring device, so from that perspective light is behaving more like a bunch of particles than a continuous wave.
    Last edited: May 15, 2016
  8. May 15, 2016 #7


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    Gold Member

    And to extend this just a bit for the OP, what's also true is that not just any "photon" can excite just any electron. That ability depends on the frequency of the wave that causes the excitation so details of the "photon nature" side of a beam of light is a directly affected by the frequency of the light wave.

    Photons are not "particles" in the classical view of particles, they are instead more accurately referred to as "quantum particles" or better still, "quantum objects".
  9. May 15, 2016 #8


    Staff: Mentor

    The same as when you throw a dice. You know a side will come come up with a probability of 1/6 but nothing happens until you throw the dice.

    QM is a theory about observations that appear here in a common sense classical world. We can predict probabilities of those observations but nothing happens until an observation actually occurs.

    With light we can predict the probabilities of observing a photon, and that is done with the wave-function (also called a probability wave) but nothing happens until it is actually observed.

    Note: What I said above isnt quite true because photons don't really have a wave-function in a usual sense. But liberties with the truth about photons is often taken in beginner texts.

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