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Homework Help: Photons emitted per sec.

  1. Mar 14, 2010 #1
    1. The problem statement, all variables and given/known data

    An LED emits a light beam with a power of 10 mW. The wavelength of the light is 420 nm. How many 420nm photons are emitted per sec.?

    2. Relevant equations

    E=hc/lamda is 2.95E-9

    P/E= Photons emitted per sec.


    3. The attempt at a solution

    2.95E-11 is wrong.

    2.1E16 photons per/sec. is the answer
  2. jcsd
  3. Mar 14, 2010 #2


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    Your value for E is wrong.

    h=6.626068 × 10-34 m2kg/s
    Last edited: Mar 14, 2010
  4. Mar 14, 2010 #3

    (6.626x10E-34)(3.0x10E8)=1.9878E-25 / 420nm = 4.7328E-28

    (10x10E-3) / (4.7328E-28) = 2.1129E25

    This is still not 2.1E16
  5. Mar 14, 2010 #4


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    You forgot to convert nm to m.
  6. Mar 14, 2010 #5


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    Check over your exponents in the part in red, it is not E-28.
  7. Mar 14, 2010 #6
    1.9878E-25/4.2E-7 = 4.7328E-19

    (10x10E-3) / (4.7328E-19) = 2.1E16

    Thank You!
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