When a point light source, of power E emitting   monochromatic light of wavelength λ is kept at a distance r from a photosensitive surface of work -function ϕ and area s, then number of photons striking the surface per unit time is

The number of photons released per unit timen= power  energy of each photon =w(hc/λ)These photons are spread out in all directions over an area 4πr2. The share of an area s is s/4πr2 of total  number of photons. So, the number of photons striking area s of photo-sensitive surface per unit timen′=W(hc/λ)×s4πr2=Wλs4πhcr2