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How to calculate the intensity of light, given area and mass

  1. Dec 2, 2017 #1
    1. The problem statement, all variables and given/known data

    You want to support a sheet of fireproof paper horizontally, using only a vertical upward beam of light spread uniformly over the sheet. There is no other light on this paper. The sheet measures 20.8 cm by 28.8 cm and has a mass of 1.70 g .
    (a) If the paper is black and hence absorbs all the light that hits it, what must be the intensity of the light beam?


    2. Relevant equations
    I=P/A
    A=w*h

    3. The attempt at a solution
    A=0.0599m^2
    I do not know power, so I do not know the intensity. What information does saying that the paper absorbs all the light that hits it actually give me?
     
  2. jcsd
  3. Dec 2, 2017 #2

    scottdave

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    Each photon can have momentum, as well as having energy. I'm on my phone, but you can search for momentum photons to get related information.
     
  4. Dec 2, 2017 #3

    haruspex

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    It affects the momentum conservation equation.
     
  5. Dec 2, 2017 #4
    Thanks guys! Got it!
     
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