This problem is actually an example I'm working through... 1. The problem statement, all variables and given/known data We are trying to estimate the (magnitude of the) electric field in the light waves from the sun hitting the surface of the Earth. 2. Relevant equations The Poynting vector for a sinusoidal EM wave has magnitude cε0E02/2 (this was derived before - I understand this). The intensity of sunlight at the surface of the Earth is 1300Wm-2 3. The attempt at a solution So the idea is that the magnitude of Poynting vector for a single EM wave will represent the power flow per unit area for this single EM wave at the surface of the Earth. The example then just equates this to the intensity and solves for E0. But I'm missing something. We have equated an expression for the power per unit area (Poynting vector) for a single EM wave to an expression for the power per unit area for all EM waves incident on that unit area, so we are attributing this intensity all to one wave when we shouldn't be and so are overestimating the field. Can anybody explain? Would it make more sense if the example meant we were trying to find the electric field at the surface due to all waves, not just one? Thankyou.