Reflection and refraction

When light hits a surface, some of it bounces off — reflection — and, if the surface leads into a transparent material, some of it passes through while bending — refraction. Most surfaces you look at every day do a bit of both.

Reflection follows a simple rule: the angle at which light arrives, measured from an imaginary line perpendicular to the surface called the normal, equals the angle at which it leaves. Both angles are measured from that normal line, not from the surface itself — a very common slip when first learning the law.

Refraction happens because light travels at different speeds in different materials, and that speed change bends the ray's path at the boundary. Light slows down and bends toward the normal when entering a denser material, and speeds up and bends away from the normal when leaving one — the bending always happens, in one direction or the other, whenever light crosses into a different transparent material.