1. The problem statement, all variables and given/known data When deriving the intensity of an electromagnetic wave in my textbook, there was a part I didn't quite get. It's how I = Watts/m^2 becomes I= u c My textbook gave an example of a electromagnetic wave through a cylinder, with cross-section area A. In 1 second it would have traveled length c. Now that makes sense. It then proceed with changing the cross-section area with 1 and by that derives I= u c.. ? I don't get it. 2. Relevant equations I = Watts/m^2 I= u c 3. The attempt at a solution So can anybody help me conceptualise this derivation?