if you put two one way mirrors against each other, with the mirror surfaces on the outside and window sides on the inside, in between two rooms, will the darker room still catch light from the brighter room?

Alternatively, would one window with mirror foil on both sides have the same result?

  • 1
    $\begingroup$ If, by one way mirror you mean a passive device that reflects light coming from one direction and lets it through from the other, one way mirrors don’t exist, so the question doesn’t make sense. $\endgroup$
    – knzhou
    Commented Jun 25, 2018 at 18:45
  • $\begingroup$ If you assume that your 'mirror side' is infallible at reflecting light, that would be the same as an opaque wall, so yeah, no light will be entering the dark room. And yes, you will have the same result with mirrors at both sides. $\endgroup$ Commented Jun 25, 2018 at 18:58
  • $\begingroup$ If a so-called "one way mirror" reflects 90% and transmits 10%, and if you sandwich two of those together, the assembly will transmit 10% of 10% (i.e., 1%). $\endgroup$ Commented Jun 25, 2018 at 20:35
  • $\begingroup$ P.S.: You probably want to put the metalized sides together, for two reasons: (A) to protect the metalization, and more importantly, (B) to avoid creating an accidental "infinity mirror" effect. $\endgroup$ Commented Jun 25, 2018 at 20:37
  • $\begingroup$ no light "diode" material is known so far $\endgroup$
    – lurscher
    Commented Jun 25, 2018 at 21:46

2 Answers 2


Let us name the rooms left and right. assume that right is bright and left is dark. If you place 2 mirrors back to back and both rooms are exposed to the mirror sides (assuming 100% transmission from 1 side and 100% reflection from the other side), no light will enter the dark room. It just acts like an opaque object.

There is only one configuration in which light will reach left- left is exposed to a mirror side and right is exposed to the non-mirror side


Your question itself has the answer. We typically think of a window as some medium through which light may pass to a varying degree, i.e. a stained glass window. If you are talking about a 'one way mirror' like one you might find in some Hollywood interrogation scene, then by definition some light should pass through; how else would you 'see through' the non-mirror side? The point of such a 'one-way-mirror' isn't to reflect all light- rather, it is to reflect most light while still allowing outside (or inside, in your case) viewers to see the other side.

Therefore, by definition light should be passed through from the lighter room (and vice versa.)


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