An isotropic point source emits light with wavelength 500 nm . The radiation power of the source is P=10 ~W . Find the number of photons passing through unit area per second at a distance of 3 m from the source.

Step-by-step solution

Given, =500 ~nm =500 10⁻⁹ ~m , As, aligned P & =n₀ h c n₀ & = P h c ...(i) aligned where, n₀ is number of photons per second At distance r from point source, number of photons/area/time n^ = n₀ 4 r^2 = P h c 4 r^2 [ from Eq. (i)] aligned & = 10 500 10⁻⁹ 6.6 10⁻³⁴ 3 10^8 4 (3)^2 & =2.23 10¹⁷ / m ^2 . s aligned