One-way mirror

One-way mirrors (also called two-way mirrors) work by overwhelming dim transmitted light with bright reflected light.

A true one-way mirror that actually allows light to be transmitted in one direction only without requiring external energy is not possible as it violates thesecond law of thermodynamics: if one placed a cold object on the transmitting side and a hot one on the blocked side, radiant energy would be transferred from the cold to the hot object. Thus, though a one-way mirror can be made to appear to work in only one direction at a time, it's actually reflective from either side.

 

 

 

One-way windows

One-way windows can be made to work with polarized light in the laboratory without violating the second law. This is an apparent paradox that stumped some great physicists, although it does not allow a practical one-way mirror for use in the real world.[23][24] Optical isolators are one-way devices that are commonly used with lasers.