# Number of emitted photons per second

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1. Jun 19, 2016

### Coldslooks

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. Jun 19, 2016

### tommyxu3

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. Jun 19, 2016

### Coldslooks

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 :)