For light diverging from a point source,

Consider the diagram in which light diverges from a point source (O).

Due to the point source light propagates in all directions symmetrically and hence, wavefront will be spherical as shown in the diagram. If power of the source is P, then intensity of the source will be

$I=\frac{P}{4{\mathrm{\pi r}}^2}$

where, r is radius of the wavefront at any time.