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Homework Help: Intensity question

  1. Jan 23, 2008 #1
    1. The problem statement, all variables and given/known data
    A low energy, 13-watt bulb illuminates your desk from a distance of 1.2m. You can assume that it acts as a point source and that it is 100% efficient. What is the intensity (power per unit area) at the desk (in Wmˉ²)?

    2. Relevant equations
    Velocity = Frequency * Wavelength
    Intensity is proportional to amplilude²

    3. The attempt at a solution
    I don't know how to start!
  2. jcsd
  3. Jan 23, 2008 #2

    Doc Al

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    Staff: Mentor

    Imagine the bulb as a point source sending out its energy equally in all directions. By the time the light has traveled 1.2 m, over how large an area is the total power spread? Hint: What's the area of a sphere of that radius?
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