(a) Show that the intensity I (the energy crossing unit areaper unit time)is the product of the energy per unit volume u and the speed of propagation v of the wave disturbance.
(b) Raio waves travel at a speed of 3.0x10^8m/s. Find the energy desity in a radio wave 480km from a 50000-wsource, assuming the waves to be spherical and the propagation to be isotopic.