Alaser produces 21.0 mw of light. in 3.00 hr , the laser emits 4.53×1020 photons. what is the wavelength of this laser?

Total energy =  Power x Time(in sec)

 =     = 226.8 J

Total photons = 4.53 x 10^{20}

Energy per photon =  Total energy / Total photon = 50.6 x 10^{-20} J

So, Applying the formula:

E = hc / lambda

lambda = hc / E

Planck constant h = 6.626×10^-34 J s;
c = speed of light = 3×10^8 m s-1 

 
λ = (6.626×10^-34)×(3×10^8)/ 50.6 x 10^{-20} = 0.39 ×10^-6 m= 0.39 um
 

The answer options is d