One-way Mirrors?

Most recent answer: 03/10/2011

How do one-way mirrors work?
- Helen (age 14)
Chicago, IL, USA
Standard so-called one-way mirrors really partial two-way mirrors. They reflect most but not all of the light that hits them. If the room on one side is brightly lit and the other side is dimly lit, the people in the dark side can see those on the bright side but not vice-versa.

A device which allowed the light to flow one way but not the other way would break what we call time-reversal symmetry. Light flowing one way is just the same as light flowing the other way, viewed in reverse. To a physicist "breaking time reversal symmetry" brings up certain thoughts. There's no way to do it with any sort of fancy mirrors or twisty passages or any such structures. Magnets, however, do change their direction of magnetism if you run the currents in them backward. So can magnets  help build a true one-way window? They can indeed:  Such one-way devices are often used in handling microwaves, but rarely used for visible light. If you have any trouble following the description in that link, please come back for a follow-up.

Mike W.

(published on 03/10/2011)