Question

A laser produces 20.0 mW of red light. In 1 hour, the laser emits 2.29 X 10^20 photons. What is the wavelength of the laser?

Answer 1

Using Planck's equation,

E = nhc / λ

Also, Power = Energy / time
E = 20 x 10⁻³ x 60 x 60
E = 108 J

108 = (2.29 x 10²⁰ x 6.63 x 10⁻³⁴ x 3 x 10⁸) / λ
λ = 4.22 x 10⁻⁷ m