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

Laser energy reflection

  1. Jan 5, 2009 #1

    If in a vacuum, I have a laser pointing into a mirrior wich bounces between two mirriors on the same spot, do the photons / laser energy continuously build up? If not whare does it go?
  2. jcsd
  3. Jan 5, 2009 #2
    I don't know for sure, but I imagine it's lost as heat via infrared waves.
  4. Jan 5, 2009 #3


    User Avatar
    Science Advisor
    Homework Helper

    You can't arrange a pair of (finite) mirrors so that you can fire a beam in and have it bounce back between them indefinitely.
    If you could then with real mirrors a small amount is absorbed on each bounce and so the energy would gradually be converted to heat in the mirror.

    Some types of laser are made from a pair of mirrors. Light bounces between them traveling back and forth through a laser medium, but you need to feed energy into the laser medium material to make new photons.
    One of the mirrors is usually semitransparent to let the laser light out.
  5. Jan 5, 2009 #4

    The amount of energy converted into heat in the mirrior would have to equal to the amount of energy comming from the laser. If not, the theres excess energy that has to go somewhere.
  6. Jan 5, 2009 #5
    Most of the energy would remain as photons bouncing between the mirrors, but it would loose a bit of energy every bounce-cycle to heat so the system would run out eventually. In fact, it would run out exactly when you lost 100% of the energy you put in as heat. Also, unless the mirror is absolutely flawless (which I bet is impossible), you'll loose some photons that way too as they aren't redirected perfectly to the other mirror. So it's finite, unless you had a constant supply of energy to keep producing photons, but then we're talking just hypotheticals at that point. Also, to reiterate, the other guy said you can't set up finite mirrors like that anyway and he has a lot of posts so I bet he's right.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook