The intensity or power output of a laser (P) is equal to nhf, where n is the number of photons emitted per second, h is Planck's constant and f is the frequency.
Therefore, by rearranging:
n=P/(hf)
Since we don't have frequency but rather wavelength, we can use c=f lambda (rearranged to give f=c/lambda) to give a final, more useful equation:
n=P/(hc/lambda)
One further rearrangement, though you could use the above form:
n=(P lambda)/(hc)
We need to get everything into the correct units before we apply the equation.
The power output is 2.3mW, this must be in Watts (so 2.3*10^-3 Watts).
The wavelength must be in metres not Ångströms and since 1 Ångström =10^-10 metres, then our wavelength in metres is 6370 * 10^-10.
c represents the speed of light 3*10^8 ms^-1
h is Planck's constant - 6.63*10^-34 Js
So finally, n=(2.3*10^-3*6370*10^-10)/(6.63*10^-34*3*10^8)
n=7.37*10^15 s^-1
