# Problem: The watt is the derived SI unit of power, the measure of energy per unit time: 1W = 1J/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 69 minutes in length?

###### FREE Expert Solution

We’re being asked to determine the number of photons that strike the CD surface during the playing of a CD in a length of 69 minutes.

Recall that the energy of a photon (E) is given by:

where:

h = Planck’s constant (6.626 × 10–34 J • s)

v = frequency (in s–1)

Also, recall that the frequency (v) and wavelength (λ) are related:

where:

c = speed of light (3.0 × 108 m/s) ###### Problem Details

The watt is the derived SI unit of power, the measure of energy per unit time: 1W = 1J/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 69 minutes in length?