Physics - Wave Propagation 01

The term [4π(6)^2 m^2 ] is the surface area of an imaginary sphere with radius 6 m and centred at the source.
[surface area of sphere = 4π x radius^2]

Intensity is defined as the energy falling on unit area in unit time. A 100 w light source gives 100 J in 1 sec. Assume no energy is lost up to 6 m, this amount of energy will spread over the surface of a sphere of radius 6 m with its centre at the position of the souce. Thus, the energy on unit area on the sphere surface = 100 J/4π(6)^2 m^2 = 100/4π(6)^2 J/m^2

But this amount of energy is emitted in 1 sec, hence intensity
= 100/4π(6)^2 J/s-m^2 = 100/4π(6)^2 w/m^2