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Number of emitted photons per second

  1. Jun 19, 2016 #1
    1. The problem statement, all variables and given/known data
    The average wavelength that LED emits is 500nm. The electrical power of the LED is 300mW and the efficiency of turning energy into light is 66%. Number of emitted photons in unit time is?
    2. Relevant equations
    ##E_f=hcf##
    ##\frac{Pt}{E_f}=number of photons##
    3. The attempt at a solution
    After calculating the energy of one photon i am to find the overall energy that is transformed into light. The energy is ##300x10^{-3}J## but how am i to include the efficiency? Just multiply the Energy with 0.66 and use that in the equation? The answer is suppose to yield ##5x10^{17} per second##
     
  2. jcsd
  3. Jun 19, 2016 #2
    First, this must make no sense. The correct one should be ##E=hf.##
    Since you can get the energy of a photon, then given the amount of light you can calculate the number of the photons. Like what you said, the energy transferred to light per second is ##300mJ\cdot 66\%.##
     
  4. Jun 19, 2016 #3
    Yeah, your right, i havent seen that ive put ##c## there, thanks.
    Well then, ##\frac{E_t}{E_f}=\frac{198*10^{-3}J}{hf}##
    ##f=\frac{c}{w}=6*10^{14}## yes the rest is clear thanks :)
     
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