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Homework Help: Divergence angle

  1. Oct 25, 2009 #1
    1. The problem statement, all variables and given/known data
    A tiny laser beam is directed from earth to moon. If beam's diameter is 2.50 m at the moon, how small much the divergence angle be for the beam? The distance of moon from the earth is 3.8x10^8 m

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 25, 2009 #2


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    Hi mars shaw! :wink:

    (try using the X2 tag just above the Reply box :wink:)

    Show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:
  4. Oct 27, 2009 #3
    s=diameter=2.50 m
    r=distance of moon from earth=3.8x10^8
    theta=2.50/3.8x10^8 = 6.6x10^-9 rad
    I got the correct answer but I am confuse about diameter that how it can be taken as "s"?
    Kindly explain.
  5. Oct 27, 2009 #4


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    Hi mars shaw ! :smile:

    Because you imagine the line from your eye to the bottom of the moon as being length r, and the diameter (s) of the moon as being at right-angles to that line, so the tangent of the angle is s/r.

    (and if you're not happy about that, but you'd rather use the radius, and the line to the centre of the moon, you'll get the same result: the tangent of half the angle is (s/2)/r :wink:)
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