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QM light reflection question.

  1. Jun 14, 2013 #1
    From this video of a Feynman lecture:

    At 39:30 he (Feynman) draws a line from the eye to the mirror, and then continues the line through the mirror, to represent where the image appears to come from (it appears to come from behind the mirror). Here he is showing the classical way of thinking of a light ray. Here the angle of incidence and the angle of reflection are the same.

    Now as he explains the quantum mechanical view, at 41:40 he draws a path located at the left side of the mirror, as one of many paths the light can take (one of many possible probability amplitude contributions).

    He continues to draw more paths that the light might take, but now I have my question. If the light might have taken another path than from the center of the mirror (where angle of incidence and angle of reflection the same), it seems to me that the apparent position of where the light comes from, would be shifted, and therefore wouldn't that cause our view of the image to be fuzzy or blurry?

    I hope my question is clear. I am not questioning the math of quantum mechanics. I understand that it is quite accurate. I just don’t see the logic in the interpretation of this particular example.
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Jun 14, 2013 #2


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    I am by no stretch particularly knowledgeable on this but I think it likely that this is one of those "sum of all possible paths" things where for practical purposes, all the other paths cancel each other out.
  4. Jun 14, 2013 #3
    The other paths don't always cancel out. There are many paths with a non-zero probability.

    In fact, later in the video he demonstrates that you can take out the center of the mirror and still receive an image. The "sum of all possible paths" that you mentioned is changed such that much of the light from the left edge no longer gets cancelled out.
  5. Jun 14, 2013 #4


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    It's important to realize that these concepts did not originate with quantum mechanics, or with Feynman. Classical optics was being studied with the aid of Huygens principle a hundred years earlier. Later, Feynman and others realized they could also be usefully applied to QM. The present question in fact relates to classical optics and not QM.

    Yes, this is diffraction, which for a finite mirror is always present. The width of the diffraction pattern depends on the wavelength of the light and the size of the mirror.
  6. Jun 14, 2013 #5
    So the effect of the contribution for various paths is a diffraction pattern, instead of a blurring of the image.

    Thanks for the replies folks...
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