For using Galilean transformation, I have to assume that speed of light w.r.t. ether frame is c. W.r.t. ether frame, E = E0 eik(x-ct) W.r.t. S' frame which is moving with speed v along the direction of propagation of light, E' = E0 eik(x'-c't') Under Galilean transformation, x' = x-vt, t' = t, c' = c -v So, (x'-c't') = x -ct thus, E' = E i.e. under the Galilean transformation, em wave equation is invariant. Is this correct? I am asking it here instead of creating a new thread, because I think the topic is similar. If it is not allowed, sorry for it.