Awatt is a unit of power or 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 0.120 mw. how many photons strike the cd surface during the playing of a cd 78 minutes in length?

n = 2.208x10¹⁸ photons

Explanation:

The energy of a photon( an elementary particle) is given by the equation:

E = nxhxf

Where n is the number of photons, h is plank constant (6,62x10⁻³⁴ J.s), and f is the frequency. Knowing that the power level is 0.120mW (1.2x10⁻⁴ W), the energy in J, for a time of 78 min (4680 s)

E = 1.2x10⁻⁴x4680 = 0.5616 J

The frequency of a photon is its velocity ( c= 3x10⁸ m/s) divided by its wavelength, which is 780 nm = 780x10⁻⁹ m

f = 3x10⁸/780x10⁻⁹

f = 3.846x10¹⁴ s⁻¹

Then, the number of photons is:

0.5616 = nx6,62x10⁻³⁴x3.846x10¹⁴

n = 2.208x10¹⁸ photons.