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B Light through vaccuum?

  1. Jan 21, 2017 #1
    I asked myself this question : If you have a hollow sphere with reflective surface on the inside and send light into it what happens to the light? and found a number of places where the question has been asked and answered.

    One answer : talked about electro magnetic field being everywhere even in vaccuum before any light travels through it and went on to talk about the nature of light. This led me to another question.

    If light is sent off through vaccuum space to another point, is it possible to detect that light from a point not inline with the path of light? Does the light 'disturb' the electro magnetic field in a way that's detectable?

    I have only high school physics from 40 years ago but I try to understand. I have no doubt that I use the wrong terms and concepts to formulate the question.
  2. jcsd
  3. Jan 21, 2017 #2


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    Staff: Mentor

    In real life, it will bounce back and forth a few thousand times before being dissipated to nothing. If you assume perfect reflectivity, it would bounce back and forth forever.

    For the second question; light is only detectable when it hits a detector. You cannot detect it otherwise.
  4. Jan 21, 2017 #3
    Thank you.

    What happens if the sphere is very cold or very hot?

    edit add: what if you send light into the sphere faster than it dissipates?
    Last edited: Jan 21, 2017
  5. Jan 21, 2017 #4


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    not sure how much heat or coolness of the material would affect absorption ?

    you need to understand that any object above 0 K (zero Kelvin) is radiating EM

    faster ? .... EM has a fixed speed in a given meduium
  6. Jan 22, 2017 #5
    What I mean is if the amount of light entering the sphere is greater than the rate at which it dissipates. Even that sound a bit odd but surely what I'm trying to say is clear.?
  7. Jan 22, 2017 #6


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    Typically, a mirrored wall will absorb some percentage of the inbound light and reflect the rest. Accordingly, if you send in more light, the dissipation will increase in proportion. Ultimately, 100% of the incoming light will be lost regardless of how perfect the mirrors are, how they are angled or how fast you inject light into the enclosure. Inject more light and you lose more at each bounce. Improve the mirrors so that less is lost at each bounce and you get more bounces.

    Of course, If you inject light faster than you can drain heat from the mirrors as the light is absorbed, you will eventually melt or vaporize the mirrors.
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