1. The problem statement, all variables and given/known data Q. Explain why the intensity of a sound or light wave is proportional to A^2, where A is the amplitude of the wave. 2. Relevant equations S = (1/μ_0)ExB ... Poynting vector S = (1/μ_0)EB B = E/c E = Asin(kx - ωt) 3. The attempt at a solution Hi everyone, So, for light, I used the pointing vector as above, substituting in B = E/c to get S = (1/cμ_0)E^2 and then E = Asin(kx - ωt) to get S = (1/cμ_0)A^2sin^2(kx - ωt) And I used I = S_average So I = (1/cμ_0)[A^2sin^2(kx - ωt)]_avg From which we can conclude that I α A^2. Is this correct? As regards sound, I really don't know what to do, as it's not an electromagnetic wave - is it still possible to use the Poynting vector? Please help!