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Homework Help: Electromagnetic Intensity

  1. Apr 6, 2009 #1
    1. The problem statement, all variables and given/known data
    A helium-neon laser of the type often found in physics laboratories has a beam power of 3.50 mW at a wavelength of 633 nm. The beam is focused by a lens to a circular spot whose effective diameter may be taken to be equal to 2.00 wavelengths. Calculate the intensity of the focused beam.

    2. Relevant equations

    I = P / 4\pi r^2


    3. The attempt at a solution

    I = P / 4\pi r^2


    P = 3.50mW
    I don't know what the radius is. From the problem, I used 633nm as the radius because the diameter = 2*(633nm), I used 633nm as the radius, but after using that value the answer is wrong. If someone can tell me the radius, I think I could get this problem. Thanks
  2. jcsd
  3. Apr 7, 2009 #2


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    Science Advisor
    Homework Helper

    Hi ganondorf29! :smile:

    (have a pi: π :wink:)
    No, that's the wrong formula …

    as you can see, it's the total power divided by the surface area of a whole sphere of radius r …

    it applies when the beam is all-round, so the intensity is P/4πr2 , and the power through an area A (at distance r) is PA/4πr2.

    In this case, the total power is confined to the beam, and it all goes through the "effective circle", so the appropriate formula for intensity is … ? :smile:
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