Physics Help Forum How to proof the reflected wave of a Plane Wave is a Plane Wave?

 May 30th 2013, 10:05 PM #1 Member   Join Date: Nov 2010 Posts: 40 How to proof the reflected wave of a Plane Wave is a Plane Wave? I have been looking around, I have not manage to find the reflected wave due to a Plane Wave hitting a flat boundary ( say xy plane) is a Plane Wave also. For example, in the case of normal incidence where $\vec E_I(z)=\hat x E_I$, the book just gives $\vec E_R(-z)=\hat x R_R$. I am trying to proof the reflected E wave in normal incidence is in x direction.