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Simple reflection thought experiment

  1. Jan 26, 2005 #1
    I am going to start out with a simple thought experiment:

    A particle is moving towards a wall at a 90 degree angle. The wall is perfectly smooth; therefore, when the particle bounces off, it will be traveling along the same path as it did when it came in.

    Does the ball ever come to a complete stop? To me, it makes since that it would have to completely stop before it can turn around and come back.

    If you say yes:
    What about light... if light reflects off of a mirror at a 90 degree angle, then it can't stop, so how does it change directions?

    If you say no:
    Please explain how the particle does not stop at the instant it hits the wall.

    Thanks for your help!

    -Matt Tucker
  2. jcsd
  3. Jan 27, 2005 #2
    Sorry! I just realized that the ball here should be listed as partical.

    I would think that somebody would have an answer to this question...
  4. Jan 27, 2005 #3


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    It disappears, and at that very moment, a new one appears with opposite velocity ? For instance ?

  5. Jan 27, 2005 #4
    Problem is you are looking at two phenomena that maybe look alike but that are fundamentally different in nature. The instant change in impulse will be valid for the ball by applying classical mechanics. However for the photon, a QM-description is required. In this case you cannot look at the photon as a billiard ball. The incident photon is NOT the same as the reflected photon and the answer of vanesch illustrates that. It is indeed what happens: absorption and emission of the photons at the wall...

  6. Jan 27, 2005 #5
    At one point in time the ball indeed stops. It's initial kinetic energy is now in the form of potential energy by the elastic deformation of the ball and wall. When light hits a wall it is absorbed, and it excites the atoms making up the wall. The excited atoms 'fall' down from their excited states to their ground state while re-emiting the energy in the form of radiation. So the light does not stop, but is absorbed and re-emitted.
  7. Jan 27, 2005 #6
    Thank you very much, that makes much more since!
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