Mirror (optics)

A mirror is a surface, typically flat, that reflects back a percentage of the light striking its surface. Typical mirrors reflect light away from the mirror at angle equal to, but opposite in sign, to the angle of the incident light relative to a normal vector. However, by cleverly taking advantage of the non-linear polarizability of some materials, one can produce a phase-conjugate mirror that only reflects light exactly back to the source. Such technology has been used in the past for private communication between airplanes and submarines at sea, where the submarine can turn the mirror on and off, in a morse code-like fashion, allowing secret communication and radio silence.