# Find number of photons emmitted per sec?

Hi, I have a question asking me to find the number of photons emmitted per second. I am given the information that the Gallium Arsenide diode laser emit 0.9 mW of power inside a CD player. The bandgap of the Gallium Arsenide is 1.42 eV. What equation is used to find this?

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P = E/t (power, energy, time)
E= n (1 photon energy)

n: piece

Defennder
Homework Helper
How come you're not given the wavelength of the emitted photons?

EDIT: Ok, I assume it's because the energy of each photon emitted would be equivalent to something which is already given in the question.

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Where is the bandgap taken into account?

I need the bandwidth

Redbelly98
Staff Emeritus
Homework Helper
Two pieces of information are needed to solve this:

1. The amount of energy emitted per sec.
2. The amount of energy per emitted photon.

Units aside, this information is given directly in the problem statement. The problem is to figure out how to combine the 2 numbers to get the number of emitted photons per second.

Again, how to combine
(energy) / (sec)
and
(energy) / (photon)

in order to get

(photons) / (sec)

p.s. some unit conversions are required here.

Okay so I get:

0.0009 Joules/second

and

(1.42 eV) * 1.602 x10^-19 = 2.27 x 10^-19 Joules - Is this correct? Is this the energy per photon?

Does (energy/photon)/(energy/second) not give photons/second.

Is that how the problem is solved?

Redbelly98
Staff Emeritus
Homework Helper
Okay so I get:

0.0009 Joules/second

and

(1.42 eV) * 1.602 x10^-19 = 2.27 x 10^-19 Joules - Is this correct? Is this the energy per photon?
Yes.

Does (energy/photon)/(energy/second) not give photons/second.
No, but you are close.

You wrote:

$$\frac{(\frac{\mbox{energy}}{\mbox{photon}})}{(\frac{\mbox{energy}}{\mbox{second}})}$$

How do the units work out in what you wrote?

The only alternative I can think of is the Joules per second divided by the Joules per photon. This gives 3.96 x 10^16 Photons per sec. Is this correct?

Redbelly98
Staff Emeritus
Homework Helper
You have the right idea, but entered the numbers into your calculator incorrectly.

Oh yes, 3.96 x 10^15. Is this the correct final answer. Thanks again for all your help.

Redbelly98
Staff Emeritus