In textbook on optics by Pedrotti (and I think Fowles does the same) on page 493 it is said that: kr=krr=ktr from which follows (k-kr)r=(k-kt)r=(kr-kt)r=0, where k, kr and kt are wave vectors of incident, reflected and transmitted wave. Than Pedrotti concludes that kr and kt must lie in a plane determined by k and r. Must they? Take, for example this: k=(-1,0,1), r=(1,0,0), kr=(-1,5,3), kt=(-1,2,1). If we put this vectors in the equation above, we will get 0, but kr and kt don't lie in the plane determined by k and r, which is y=0 plane. So I conclude that Pedrotti is wrong. Am I right or wrong to conclude that? If wrong, why?