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#
