the law of reflection

Stand in front of a flat mirror and raise your right hand: the image raises what looks like its left hand, yet it copies your every move exactly. Bounce a ball straight down onto the floor and it comes straight back up, but throw it in at a slant and it bounces off at the same slant on the other side. Light does the same when it hits a smooth surface. The law of reflection is the simple rule that tells you which way a ray of light bounces. It answers the question: if I know the direction light comes in, which direction does it leave?

To state it precisely we first draw the normal, an imaginary line drawn perpendicular (at a right angle) to the surface at the point where the light lands. The angle of incidence, theta_i, is measured between the incoming ray and the normal, and the angle of reflection, theta_r, is measured between the outgoing ray and the normal. The law of reflection says the angle of reflection equals the angle of incidence, theta_r = theta_i, and that the incoming ray, the outgoing ray, and the normal all lie in the same flat plane. Both angles are always measured from the normal, not from the surface itself.

This one rule governs every mirror, still pond, and polished floor. When the surface is smooth, parallel rays stay parallel after bouncing and you get a clear image, this is called specular reflection. When the surface is rough, like paper or a wall, each tiny patch still obeys the law but the patches face slightly different ways, so the rays scatter in all directions, this is diffuse reflection, and it is why you can read this page from any angle without it acting like a mirror.