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

What happens to the freq of a photon after encountering double slit?

  1. Sep 14, 2013 #1
    1. (How) Is the frequency of a photon effected after encountering a double-slit or a half silvered mirror (say in a Mach-zehnder)?

    2. Are particles such as photon, electron, assumed to be entangled, at all points in time-space, with something or the other?
  2. jcsd
  3. Sep 15, 2013 #2
    1. Photon frequency is not changed in double slit experiment, nor during reflection.

    2. Particles are entangled with other particles, if they interacted.
  4. Sep 15, 2013 #3
    Thanks mpv.

    So, thus the energy of the photon is same throughout? Prior and post double-slit or mach-zehnder
  5. Sep 15, 2013 #4


    User Avatar

    Staff: Mentor

    For all practical purposes, yes.

    If you want to get really really really really picky about it, when a photon changes direction and therefore changes momentum, the apparatus must recoil in order to conserve momentum. The apparatus therefore gains some kinetic energy, which must come at the expense of some of the photon's energy. Therefore the photon's energy must be very very very very slightly less after reflection. But this is negligible for all practical purposes. If anyone has a reference to experimental results that actually observe this difference, please post it!

    I mention this only because based on previous experience here, there is about a 50% probability that someone will bring this up anyway. :devil:
  6. Sep 15, 2013 #5
    Um, I don't think this is right. You are right in pointing out that a mirror should "recoil" when it reflects a photon, but in a Mach-Zehnder interferometer, we treat the mirrors as rigidly fixed with one another so there is net cancellation of the forces acting on the overall mirror apparatus. For example, if we shoot a laser into a half-silvered mirror at the top left corner of a Mach-Zehnder interferometer, the recoil on the top right mirror would be equal and opposite to the recoil on the bottom left mirror, and if we rigidly attach them, then the recoils cancel out.

    This is a sort of funny way to describe what happens since the mirrors aren't perfectly rigidly attached (in reality they are attached to a lab bench which is in principle somewhat flexible), but if we use your suggestion, then a mach-zehnder interferometer must continually grow (this is silly) or, if you think the mirrors somehow remain stationary while changing the photons' energy, that would be akin to claiming that a driven oscillator will find an equillibrium oscillation with a frequency that is slightly less than the driving frequency. (Of course this is mathematically impossible. http://en.wikipedia.org/wiki/Damped_harmonic_oscillator#Sinusoidal_driving_force) So the better way to think about an MZ is that the mirrors are rigidly attached to one another.

    [If you wanted to go one step further in thinking about this, you could think of the flexible lab bench as some sort of spring joining the two mirrors, and the first few photons hitting the mirrors would cause the lab bench to stretch, and after a long enough stream of photons arrives, the lab bench will reach some equilibrium stretching such that it supplies an equal and opposite restoring force on the mirrors. The equilibrium configuration would keep the mirrors stationary, just as if they were rigidly attached.]
    Last edited: Sep 15, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook