## Physics: Principles with Applications (7th Edition)

When light travels from a fast medium (low index of refraction n) to a medium where it slows down (relatively higher index of refraction), it bends toward the normal. The opposite happens when it travels from a high-n slow material to a lower-index fast material: it bends away from the normal. Call the materials, from left to right, A, B, and C. The light ray passes from material A to material B and bends toward the normal. This means that $n_{A} \lt n_{B}$. The light ray then goes from B to C, and bends away from the normal. This means that $n_{C} \lt n_{B}$. We notice that it bends, but not so far that the ray ends up parallel to the original ray. Therefore $n_{A} \lt n_{C}$; Light travels faster in A than in C. If light traveled faster in C than in A, the ray would have bent even farther away from the normal.