This is known as an optical isolator. They are used in high power laser systems to prevent reflected light from propagating backward from the high power end through the amplifiers to the low power end, and causing damage. -- Gene From: Warren D Smith <warren.wds@gmail.com> To: math-fun@mailman.xmission.com Sent: Friday, June 12, 2015 1:22 PM Subject: [math-fun] one way window for light "One way mirrors" seen in police interview rooms on TV, aren't really. Light goes both ways through them, it is just that there is a lot less light coming from one side, since it is a dark place. The other side is brightly lit; and then if you partially-silver the window the scattering and partial reflection tends to overwhelm the transmission from the dark side, making it *appear* to be a mirror from the bright side. But our recent discussion re the "Faraday effect" being a "loophole" in the time-reversibility claims for Maxwell equations, made it occur to me: we really can make a one-way window for light. ---------------------xxx----------------------- ----light->>----A---F----B----light->>---- ---------------------xxx----------------------- let A be a polarizer, F is a faraday rotatory cell containing magnetic field (generated by cylindrical solenoid xxx surrounding F) pointed rightward, and B is another polarizer whose angle is 45 degrees to A, selected to transmit the 45-rotated linearly-polarized light from A as rotated by F (length of F and magnetic field strength selected to make the rotation angle 45). Result: light transmits thru left-to-right, albeit halved in power since only one polarization is transmitted. But if you try to go backward, then the Faraday F will rotate 45 degrees causing the plane-polarized light from B now to be orthogonal to A, and transmission is blocked. Note this does not work with sugar solution doing the rotating instead of Faraday. Then the backward rotation would undo the forward rotation and transmits equally both ways. Nor would any set of dielectric, partially silvered, polarizing, rotating, etc slabs work -- by time reversibility of Maxwell equations. Faraday effect based on EXTERNALLY applied magnetic field, is crucial to make it work.