Suppose a light source at A is in a medium in which light travels at a speed v_1 and that point B is in a medium in which light travels at a speed v_2 (see figure). Using Fermat's Principle, which states that light travels along the path that requires the minimum travel time, show that the path taken between points A and B satisfies sin(theta_1) / v_1 = sin(theta_2) / v_2?
