Question

Ultraviolet light emits a total of 2.5 × 10^–17 J of light at a wavelength of 9.8 × 10^–7 m. How many photons does this correspond to?

Answer 1

Applying Planck's equation:

E = nhc/λ, with n = number of photons, h = Planck's constant, c = speed of light and λ = wavelength

2.5 x 10⁻¹⁷ = (6.63 x 10⁻³⁴)(3 x 10⁸)n / (9.8 x 10⁻⁷)
n = 123 photons