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Homework Help: Another SAT II Physics question: optics

  1. May 31, 2008 #1
    [SOLVED] another SAT II Physics question: optics

    1. The problem statement, all variables and given/known data
    A point source light which has an intensity level of 10 candle power at a distance of 2 m will have an intensity at 4 m of
    A) .5 cp
    B) 1 cp
    C) 2.5 cp
    D) 5 cp
    E) 10 cp

    2. Relevant equations
    i don't really know...my book doesn't really address this
    i don't even know what a point source light does.

    3. The attempt at a solution
    i tried to set up a proportion, but i got 20 as my answer, which is obviously wrong!

    anything to start me off would be greatly appreciated!
  2. jcsd
  3. May 31, 2008 #2
    You need to know the relationship between distance and intensity.

    It is basically,

    [itex] $ I \propto \frac{1}{d^2}$ [/itex]

    With that, you should be able to solve your problem.
  4. Jun 1, 2008 #3
    oh okay;;
    so since the distance was doubled, the intensity would decrease by a factor of 1/4,
    and 10 x (1/4) = 2.5 cp
  5. Jun 1, 2008 #4
  6. Jun 1, 2008 #5
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