A point source of light of power P and wavelength λ  is emitting light in all directions. The number of photons present in a spherical region of radius ‘r’ to radius 2r with centre at source, is

Intensity at a distance x from point source is given by  I=P4πx2   consider an elemental shell of radius x and thickness dx, in this region, the energy contained is dE=Ic×4πx2dx=Pdxc  Let dn be the number of photons in this elemental shell, then dn×hcλ=dE=Pdxc           ⇒ dn=Pλdxhc2 Total no.of photons in the shell of inner radius r and outer radius 2r is n=∫dn=∫r2rPλdxhc2=Pλrhc2