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Prism gains mass due to light!

  1. Aug 8, 2016 #1
    1. The problem statement, all variables and given/known data
    Figure 1.1 shows an equilateral glass prism illuminated by a 100 W laser beam of wavelength λ = 600 nm. The refractive index of the glass of the prism is 1.50 at λ = 600 nm. The path of the light in the prism is parallel to the base of the prism. The momentum of a photon is h/ λ. Calculate the change in weight of the prism when the beam is switched on.( basically an equilateral triangle and a laser parralell to its base)

    2. Relevant equations

    3. The attempt at a solution
    I tried something which involved giving the photon a mass say M. and calculating the number of photons per unit time. I'm not sure how to continue.
    I'm basically practicing for a contest but i don't have solutions available only questions
  2. jcsd
  3. Aug 8, 2016 #2


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    Hi Rohan, :welcome:

    Photons are known to have no mass. But they do have momentum. Changing momentum requires force. Note the exerise doesn't ask for the change in mass of the prism, only for the change in weight...:rolleyes:
  4. Aug 8, 2016 #3
    I tried again. This time I'm trying to use forces. As it enters the prism i can calulate the change in momentum of the photons. Then I can relate the change in momentum to the wavelngths. Then I can calculate a value for the Force by calculating the number of photons per unit time from the power. However i can't seem to see why the Force Has to be virtical or am i missing something? im quite rusty on this topic
  5. Aug 8, 2016 #4


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    Light comes in moving slightly upwards and comes out moving slightly downwards. That's a change in vertical momentum.
  6. Aug 8, 2016 #5
    I have reached the abyss of stupidity! Thanks
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