# Number of Photons inside a Laser Cavity

HuskyLab
<< Mentor Note -- thread moved from the technical forums, so no Homework Help Template is shown >>

Let's say you have a laser cavity with two mirrors at either end, one is considered 100% reflective, the other 99.9%, so that a wave beam is emitted through this lower reflectivity mirror.
I know:
-The length of the laser cavity
-The reflectivity of the mirrors
-The power and wavelength of the emitted wave beam

I am asked to determine the number of photons present in the cavity (I presume average considering they are continually being absorbed and re-emitted). I am having a hard time understanding how to even go about this with the, at least what seems to me, limited information.

I thought about working backwards, if let's say the output mirror has a reflectivity of 99.9%, then only 0.1% of the photons inside have been emitted so N(out)/0.001 would give the average number of photons inside the cavity? I feel like I'm missing something. I know that the photons interfere to create standing waves some integer multiple fitting the cavity length.

Mentor

HuskyLab
By my teacher.

Mentor
By my teacher.
In that case, we're going to have to ask you to post this question to the "Introductory Physics Homework" subforum instead.

[Edit: looks like Berkeman beat me to it]