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Number of emitted photons per second

  • #1

Homework Statement


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?

Homework Equations


##E_f=hcf##
##\frac{Pt}{E_f}=number of photons##

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##
 

Answers and Replies

  • #2
240
42

Homework Equations


##E_f=hcf##
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\%.##
 
  • #3
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\%.##
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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