The second law of reflection

The first law tells you how big the angle is. The second tells you where the light goes. Activity 10.5 finds it.

Use the same set-up, but lay a stiff sheet of chart paper on the table so that part of it sticks out past the table's edge. Shine the beam onto the mirror and look for the reflected beam on the part of the paper that sticks out. You can see it lying along the paper.

Now bend the sticking-out part of the sheet down over the edge. Can you still see the reflected beam on it? No: it disappears. Flatten the paper again and the beam is back.

Why? While the paper is flat, the reflected beam lies along it, in the same flat surface (plane) as the incident beam. Bending the paper makes a new, tilted plane. The light doesn't bend with the paper: it carries on in the original plane, so there's no paper there to catch it.

So the second law of reflection is: the incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.

This is true however the light comes in: two rays hitting the mirror at the same point share the same normal, and each one's reflected ray stays in the plane of that ray and the normal.