Let's say we have the problem: A 100W lamp emits light in all directions. Assuming that the lamp is a point source, calculate the intensity of the radiation 1m away from the lamp. The surface area of a sphere is :4*Pi*r2 intensity = power/cross-sectional area The answer is intensity = 100w/4Pi = 7.96 Now what confuses me is that the answer is derived by intensity = power/surface area of the shape the wave makes when it spreads out. Shouldn't cross-sectional area be the cross-sectional area of a sphere? I don't see how 4*Pi*r2 is the cross-sectional area in this case.