Plane mirror

A plane mirror has a flat reflecting surface. The law of reflection sends incident unit direction $\mathbf{d}$ to $\mathbf{d}-2(\mathbf{d}\cdot \mathbf{n})\mathbf{n}$, where $\mathbf{n}$ is its unit normal. It forms a virtual image of an object the same perpendicular distance behind the reflecting plane as the object is in front. Two successive plane mirrors whose planes meet at angle $\gamma$ rotate the ray geometry through $2\gamma$ about their intersection.