Imagine a light ray as it passes from one material to another (as in Fig. 24.11) and considers the electric and magnetic fields associated with these rays at the boundary between the materials. At (i.e., very near) the boundary, the component of the electric field directed parallel to the boundary must be the same on the two sides of the boundary. Use this fact to argue how the frequency of the light wave must change (or not change) when you cross the boundary