a. Derive an expression for 21 , the wavelength of light emitted by a particle in

a. Derive an expression for λ_2→1, the wavelength of light emitted by a particle in a rigid box during a quantum jump from n = 2 to n = 1.
b. In what length rigid box will an electron undergoing a 2 → 1 transition emit light with a wavelength of 694 nm? This is the wavelength of a ruby laser.