Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

When light moves what moves

  1. Apr 6, 2012 #1
    Here's a question from my textbook. Since there are no calculations I put this in this forum rather than the hmwk forum.

    When light (or other electromagnetic radiation) travels across a given region, what is it that oscillates? What is it that is transported?

    Here is the answer provided:

    Energy moves. No matter moves. You could say that electric and magnetic fields move, but it is nicer to say that the fields stay at that point and oscillate. The fields vary in time, like sports fans in the grandstand when the crowd does the wave. The fields constitute the medium for the wave, and energy moves.

    I thought it was the photon that moves. Maybe I'm thinking of the photon as a particle when it's not really a particle but only has particle like properties.
     
  2. jcsd
  3. Apr 6, 2012 #2
    Okay. This involves a little quantum physics. I might not be qualified to answer this question, as I only have a basic understanding of it, but here goes.

    The photon isn't "really" a particle or a wave. Or it's both. In some experiments, it shows particle-like properties, and in others, it appears to show wave-like ones. (In fact, from observations, in some experiments it acts exactly like a particle and in others, exactly like a wave.) I'd imagine that in this experiment, it would exhibit wave-like properties as opposed to particle-like ones.

    This sound confusing? Welcome to Quantum Mechanics.
     
  4. Apr 6, 2012 #3

    DaveC426913

    User Avatar
    Gold Member

    It's a particle, it's just a massless particle.

    Whovian's answer is a good description.

    Thing to remember is that all words (such as "particle") are approximations and analogies. No amount of words can properly describe anything subatomic. What describes them is the formulae. Words are simply metaphors to help us visualize things. "The photon is like a particle". Like any analogy, it can be carried too far.
     
  5. Apr 6, 2012 #4

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    From the "electromagnetic wave" point of view (as opposed to the "photon" point of view) the wave is changes (increases and decreases) in the strength of the electric and magnetic fields (or the "electromagnetic" field), not actual motion of anything.
     
  6. Apr 6, 2012 #5
    All the above sounds good.....
    electromagnetic waves carry energy in a vacuum...in 'nothing'....there is no background material, no mass,no atoms, no particles, required, just space and time; with sound waves in contrast, energy is also transferred but requires a medium, like air or water or metal...some mass, some particles, to carry the energy.

    When light travels in a medium, like glass, things get more complicated.....that's another discussion.

    Also, it should be noted that while not requiring a material medium, light CAN cause a medium to gain energy....the photoelectric effect is such a situation in which incident light can set orbital electrons into different energy levels.....
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook