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There is a conference hall , glass door parallel to my desk. whenever the door is open, I can see reflection of images perpendicular to the viewing angle. But when the door is closed, I can’t see any reflection but see thru.

Why transparent glasses acts as a mirror viewed at certain angle?

One thing I noticed is, the reflection is seeing when the back side of the glass is darker than its surroundings.

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    $\begingroup$ possible duplicate of Why does a window become a mirror at night? $\endgroup$ – Pranav Hosangadi Feb 11 '15 at 8:49
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    $\begingroup$ Check the Wikipedia article on Fresnel equations which has a plot of surface reflectivity versus incidence angle. These equations, though, only give phenomenological explanation. On the physical side, I believe that at large incidence angle the electromagnetic wave cannot polarize the glass properly to induce propagation of the electromagnetic wave. $\endgroup$ – gigacyan Feb 11 '15 at 10:13