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Homework Help: Amplitude and intensity

  1. Feb 21, 2016 #1
    1. The problem statement, all variables and given/known data
    The light from a bulb shines equally in all directions. If 20W of light is given off, what will the intensity be 12m from the lamp to 2 significant figures? (Consider the shape of the region illuminated if the light hits this surface after travelling 12m in all directions.)

    2. Relevant equations
    3. The attempt at a solution
    20 / pi * 122 = 0.044 W/m2. Incorrect. I suppose this value would give me the average intensity over the entire area. Halving it should then give me the furthest intensity. 0.022 W/m2, still incorrect.
  2. jcsd
  3. Feb 21, 2016 #2


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    Staff: Mentor

    What's the total area of the surface at a radius of 12 m? (hint: the surface is not a circle).
  4. Feb 21, 2016 #3
    Argh, I was considering it shining over a one-dimensional circle.
    20 / (4 * pi * 122) = 0.011, correct answer. Thanks!
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