How many photons strike the cd surface during the playing of a cd 69 minutes in length?

Part of the text of the exercise is missing.complete text: "a semiconductor laser in a cd player has an output wavelength of 780nm and a power level of 0.10mw. how many photons strike the cd surface during the player of a cd 69 minutes in length? ? "solutionthe power is the energy per unit of time; the laser power is  , and it is applied for a time t of  . from the previous equation we can find the total energy delivered by the laser during this time: then we can find the frequency of the photons of the laser light. in fact, we know the relatioship between c (speed of light), f (frequency) and  (wavelength): using  and  , we getfrom this, we can find the energy carried by one single photon of this frequency: and finally, we can find the number of photons dividing the total energy delivered by the laser by the energy carried by one single photon: