Mirra

Meet Mirra, a flat mirror who sends light away at exactly the angle it arrived.

When light hits a smooth surface, it reflects so that the angle of incidence equals the angle of reflection. Both angles are measured from the normal, the line perpendicular to the surface. The incoming ray, the outgoing ray and the normal all lie in one plane. A ray arriving at 40° to the normal leaves at 40° on the other side, as the sticker shows.

This law follows from the wave nature of light, and also from Fermat's principle that light takes the path of least time. Because every ray obeys it, a flat mirror forms an image that appears as far behind the mirror as the object is in front, reversed front to back.

Periscopes, car mirrors, the reflecting telescopes used in astronomy and laser beam steering all depend on it. Curved mirrors use the same law at each point to focus light.

Rough surfaces, like paper, obey the law too at each tiny patch, but those patches face many directions, so the light scatters diffusely and no image forms. No real mirror reflects all the light.