Figure $\quad \mathrm{P} 34.21$ shows a light ray incident on a series of slabs having different refractive indices, where $n_{1}<n_{2}<$ $n_{3}<n_{4} .$ Notice that the path of the ray steadily bends toward the normal. If the variation in $n$ were continuous, the path would form a smooth curve. Use this idea and a ray diagram to explain why you can see the Sun at sunset after it has fallen below the horizon.