Consider an isotropic light source which emits EM radiation with wavelength λ with power P0. (Isotropic = uniformly in all directions) And consider a detector with area a, facing the source which is a long distance R away.
a. Derive a formula for amplitude of the electric field at the detector.
b. Derive a formula for the number of photons per unit time that strike the detector. What is the relationship between the rate (number per second) at which photons hit the detector, and the amplimde of the E-field at the detector?