How many photons are produced by a 60-mW laser source emitting at a wavelength of 500 nm over a 12hr period

(a) 5.1 x 10^23
(b) 6.5 x 10^21
(c) 7.1 x 10^25
(d) 6.8 x 10^24
(e) 4.3 x 10^26

Energy per photon = (Planck's constant)(speed of light) / (wavelength) = (6.626 x 10^-34 J*s)(3.00 x 10^8 m/s) / (500 x 10^-9 m) = 3.9756 x 10^-19 Joules/photon

Power = 60 mW = (60 x 10^-3 J/s)
Time = 12 hrs = 43200 s
Energy = Power x time = (60 x 10^-3 J/s)(43200 s) = 2592 Joules

so...

3.9756 x 10^-19 Joules/photon times n number of photons equals 2592 Joules

(3.9756 x 10^-19 Joules/photon) x n = 2592 Joules

solving for n

n = 6.5 x 10^21 photons (b)