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:

$\overline{){\mathbf{E}}{\mathbf{=}}{\mathbf{hv}}}\left(1\right)$

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:

$\overline{){\mathbf{\lambda}}{\mathbf{=}}\frac{\mathbf{c}}{\mathbf{v}}}\left(2\right)$

where:

**c** = speed of light (3.0 × 10^{8} m/s)

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?

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