1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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 light after interference?

  1. Sep 8, 2014 #1
    Imagine a beam of light diverging from a source and being incident on a detector. Now imagine a similar set up but with one main difference. The difference is that on its journey to the detector the beam passes through a second beam such that in the overlap region the two beams interfere. Having passed through the overlap region the beam is then incident on the detector.
    My question is, if everything else apart from the prescence of the second beam is kept the same are there any differences in what can be detected?
    As far as I know at present all similar detectors would record the same results. Having passed through the overlap region the beam would emerge with the same properties as it would have if there was no overlap region. Is that correct? I will be very grateful if anyone can confirm or clarify. Thank you.
  2. jcsd
  3. Sep 8, 2014 #2
    first of all if the beam passed over other beam, both are coherent then only interference can be observed.. if the two beams which are overlapped are from different sources then there will be no interference and there will be no change what the detector will detect otherwise.

    If the 2 beams are from same source and in phase with each other then the fringes (dark and bright) can be detected...
  4. Sep 8, 2014 #3
    As we know a beam of light is made up of photons. In classical electrodynamics the Maxwell equations are linear and therefore they cannot describe any photon-photon interaction. In the visible region (photon energies of the order of a few eV) photon-photon is negligible. For gamma rays with photon energies of the order of 500 MeV a pair of photons can annihilate creating an electron-positron pair with a substantial probability. Thus, it depends on what source of beam you use.
  5. Sep 8, 2014 #4
    I think you just couldn't grasp his question. He is not asking what it is going to be appeared in the overlapped region rather he would want to know how the interaction could change the properties of the first beam having passed through the overlapped region and being incident on the same detector.
  6. Sep 8, 2014 #5
    Hi Mohammad. As PaulDirac pointed out I want to know what observed changes are there, if any, after the beams have passed through eachother. Thanks for replying.

    Hi PaulDirac. Yes, I forgot about pair production. I didn't know what to google in order to search this topic but your post suggests that "photon photon interactions" might give some useful information. I will try it later. Thank you.
  7. Sep 8, 2014 #6


    User Avatar

    Staff: Mentor

    For all practical purposes, yes. Light meeting light is a lot like what happens when you drop two stones into a body of still water - the ripples spread out in rings, and where they meet they pass through each other and continue undisturbed.
  8. Sep 8, 2014 #7


    Staff: Mentor

    Another term is "two photon physics". Note that the cross section is very small for this interaction, so the change to the beam is very small.
  9. Sep 9, 2014 #8
    Thanks Nugatory and DaleSpam my question has been answered.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook