Trapping EM radiation Is there a material which can allow light (or any other EM radiation) to pass through from one side as if it is transparent but its other side reflects light like a mirror?

 A: There is a simple argument for why this is not possible, or more specifically why the emissivity and reflectivity must always add up to one.
Suppose you have a sphere in thermal equilibrium with it's surroundings and you wrap it in your one way film. Now the sphere will cool down because it reflects all incident radiation but still emits radiation. You can now connect a heat engine to the sphere and use the temperature difference to do work, and you have a perpetual motion device.
A: Partially coated (silvered) mirrors could do the job, if you've got some radiation of high intensity. For example, a laser. The intense beam comes out through the partial-mirror after a series of constructive reflections. Wiki doesn't have a good article on one-way windows. But, Googling it can throw a lot of results. These one-way mirrors are imaginary.
Say you're in a car and I'm out. Well, most of the cars today have these semi-reflective mirrors. If I try to peek into the car from quite a distance, I can't see you. But if I come closer enough (or stick to the glass), I can see who's hiding inside. But, this shouldn't happen with a perfect one-way mirror. I shouldn't see you at all. But, you can see me completely. Hence, we conclude that this is not practically allowable as it would violate second law of thermodynamics.
(Perhaps, there is a hiding answer in - Why does a window become a mirror at night?)
A: Generally, no. Such a material would violate time-reversal symmetry.
But do a Google Scholar search for "non-reciprocal transmission" and you will find plenty of tricks with gratings, metamaterials, etc., that allow this in some limited circumstances.
