The watt is the derived SI unit of power, the measure of energy per unit time: 1 W = 1 J/s. A semiconductor laser in a CD player has an output wavelength of 780 nm and a power level of 0.10 mW. How many photons strike the CD surface during the playing of a CD 38 minutes in length?

Answer 1

The energy of each photon is
E = hc/wavelength
E = 6.626E-34 x 3E8
780E-9 = approx 3E-19 (but you need to do this and all of the other calculations better than I'm estimating).

How many seconds do we have?
Thats 38 min x (60 s/min) = about 2300 s.

power is 0.1mW = 0.0001 W = J/s
0.0001*2300s = about 0.23 J

3E-19 J/photon x #photons = 0.23 J
Solve for #photons.
Check my thinking.