A beam of light of wavelength 620 nm is incident normally on a surface

PHYSICS · Subjective · A Level · Physics · Particles and radiation · Particles and radiation

Problem

A beam of light of wavelength 620 nm is incident normally on a surface. The intensity of the beam at the surface is

25 \text{ W m}^{-2}

. How many photons are incident on the surface per unit time per unit area?