If works from the refraction of the light by a different medium so that the light goes through a focal point outside the glass and reverses itself. That bending from going through the different medium is itself a really cool principle called Snell’s Law, and here’s a great article on it:
https://plus.google.com/+TomEigelsbach/posts/HN1EMzCSjGv
via AsapSCIENCE #scienceeveryday
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