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Will intensity of this light become infinity?

  1. Feb 19, 2007 #1
    Dear Friends,

    My longtime pending doubt here...!

    When we focus a mirror on the wall we get bright spot of the light. Ok.

    Now say, there is cube 6" x 6" x 6" whose inner walls are of mirror surfaces and opaque surfaces are the outer surfaces of the cube. In the center of the cube, in the mid space, I install a lamp (say 100W electric bulb).

    There are mutiple reflections happening since all the 6 walls are facing each other. Now If I introduce an instrument to measure the intensity or brightness of the light, How much it will be? 2 times, 3 times or 4 times or infinity?

    But I feel the answer as the "intensity will be same as that of the light at the given distance". It wont get multipled. Still I want an detailed explaination with convincing reasons. Anybody to explain me please!?
    Last edited: Feb 19, 2007
  2. jcsd
  3. Feb 19, 2007 #2
    Let's say a mirror reflects x amount of light, x < 1. So we have for two opposite mirrors, for every hop,
    1: 1(light from bulb)
    2: 1(bulb) + x(light from hop 1 reflected by opposite mirror)
    3: 1 + x(from hop 2) + x*x(from hop 1)
    ∞: 1 + x + x*x + x*x*x +... = 1/(1-x)
    this is if I didn't missed anything. the rest of light, ∞ - 1/(1-x), will be consumed by mirrors and eventually melt them. unless they radiate the heat.
    Last edited: Feb 19, 2007
  4. Feb 19, 2007 #3
    Thanks for the reply. but it is not convincing because the brightness seems function of the wavelength.... even i am also not sure.
  5. Feb 19, 2007 #4
    i mean, x like integral flux reflected in all wavelengthes. oh well.
  6. Feb 19, 2007 #5


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    Staff: Mentor

    If you could get everything perfect (you can't), the intensity would continuously increase. Real mirrors have reflectivities under 100% and real lights are bigger than a single point and get hot.
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