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B Laser and Mirror Thought Experiment

  1. Jun 9, 2017 #1
    I have a thought experiment that i need a answer to. In my experiment, I am in a bathroom with a mirror along one wall. I also have a laser that emits a single file line of photons. The laser is attached to the ceiling and emits a line of photons parallel to the mirror. What property of light makes it so i can see the photons in the mirror? A normal laser that doesn't emit just a single file of photons can be seen reflected in a mirror while it's pointed parallel to the mirror. So how is this possible? How do the photons get out of the line if the laser and reflect off the mirror to my retina? How do the photons go in a different direction than the laser is emitting them? I hope i presented my question in a answerable manner, and please excuse my lack of knowledge.
  2. jcsd
  3. Jun 9, 2017 #2


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    Staff: Mentor

    There is no such thing. A laser doesn't emit a stream of photons, whether "in single file" or not, it emits light, which is to say electromagnetic radiation. The light interacts with and is scattered by the mirror, and you see the scattered light.

    Photons only come into the picture if you do detailed quantum mechanical calculations (which are completely unnecessary for this problem) of exactly how the light is scattered by the mirror.
  4. Jun 9, 2017 #3
    Precisely my question, HOW does the EM radiation get emitted from the laser and actually travel to he mirror to be reflected? Wouldn't the light wave be traveling parallel to the mirror? The light wave would have to hit the mirror, meaning it's not parallel to the mirror anymore, right?
  5. Jun 9, 2017 #4


    Staff: Mentor

    It gets scattered or reflected off something else first. A laser beam in vacuum is not visible.

    Yes, but they don't behave the way you describe. To correctly describe photons you need to use QFT.

    But as @Nugatory says, that is unnecessary for this problem. All you need to do is say that your laser beam is well collimated and then talk about the scattering of the beam. There is no need to bring in photons
    Last edited: Jun 9, 2017
  6. Jun 9, 2017 #5
    Ah, that's a very valuable piece of information I was not aware of. Thank you!
  7. Jun 9, 2017 #6


    Staff: Mentor

    Excellent! Glad to help
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