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 0.14 mW. How many photons strike the CD surface during the playing of a CD 60. minutes in length?
(0.14 mW) / (1000 mW/W) = 0.00014 W = 0.00014 J/s, which is the power of the CD player laser in the appropriate units.
(0.00014 J/s) (60 min) (60 s/min) = 0.504 J, which is the total energy delivered by the CD player laser over one hour.
E = hc/λ = (6.626×10^-34 J-s) (3.0×10^8 m/s) / (7.8×10^-7 m);
E = 2.55×10^-19 J, which is the energy of a single photon of wavelength of 780 nm.
(0.504 J) / (2.55×10^-19 J/photon) = 1.98×10^18 photons were delivered to the CD over one hour by the laser, or
(1.98×10^18) / (6.02×10^23 /mole) = 3.29×10^-6 moles of photons were delivered over one hour.