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Working out how many complete waves are sent out.

  1. Nov 14, 2012 #1
    1. The problem statement, all variables and given/known data
    A source of light emits a train of waves lasting 0.04 μs. The light has a wavelength of 600nm and the speed of light is 3×108ms-1. How many complete waves are sent out?
    a)2.0×107
    b)4.5×107
    c)2.0×1010
    d)4.5×1013


    2. Relevant equations
    f=v/wavelength (sorry, I couldn't find the wavelength symbol)

    T=1/f


    3. The attempt at a solution
    Ok, this is what I tried:
    f=v/wavelength
    f=3×108/600×10-9
    f=0.0005

    T=1/f
    T=1/0.0005
    T=2000

    then I did:
    0.04×10-6/2000

    =2×1010

    Which was so close to one of the answers; frustrating! Could somebody please explain to me what to do? It's driving me crazy. I'm finding it hard to carry on with the rest of the assignment because I keep going back to that question and staring at it, willing my brain to work!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 14, 2012 #2


    You entered this into the calculator wrong. Watch your parenthesis.

    A big clue that something is wrong is that you have a period of 2000 s. If light has a really small wavelength and a fast speed, does it make sense that it should take over half an hour for one wave cycle?
     
  4. Nov 14, 2012 #3
    A-HA!
    Thank you! I now get the answer 2×107.
    Yeah, I did look at the 2000s and think that was a tad wrong...
    I'm now torn between being pleased that I knew how to work out the answer and the dismay of not being able to use a calculator.
    Thank you again!
     
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