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Why do I sometimes see my reflection in a glass slab ? I mean I should not be seeing it as it is a transparent material only through which refraction can only happen (provided the incident angle is less than e critical angle). We can also sometimes see our reflection even if we look to the mirror with perpendicular angles of incidence. Can you explain this phenomenon?

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    $\begingroup$ Do you know that total internal reflection only happens when the incident angle is greater than the critical angle and it only happens when, in your example, you are viewing from inside the glass slab? Instead of glass slab, think about water first. $\endgroup$ – Weijun Zhou Jan 21 '18 at 18:03
  • $\begingroup$ What I mean is, critical angle is totally irrelevant here, and there is no such thing as "only refraction can happen". $\endgroup$ – Weijun Zhou Jan 21 '18 at 18:11
  • $\begingroup$ Correction: if the light is $p$-polarized then there is the Brewster's angle in which you can have refraction without reflection. But I assume we are talking about natural light here. $\endgroup$ – Weijun Zhou Jan 22 '18 at 7:56
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Glass is a material with a higher refractive index $n_G=1.5$ than air $n_A=1.0$. Therefore light is reflected at the surface of the glass slab. Its reflectivity at normal incidence is $$R= |\frac{n_G-n_A}{n_G+n_A}|^2 =0.04$$ Therefore you can see the reflection of yourself in a slab of glass. The total reflection at the back of the glass slab for high angles of incidence is irrelevant here.

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Yes, the other poster is correct, 4% of the light bounces back from the glass interface. That's the ghost of you that you see. If its dark on the other side, your image easier to see. BTW, individual photons, emitted one at a time, will also bounce back 1/25 ratio. But how and when a particular photon bounces has never been solved, if it can be at all. Both Newton and Feynman commented on this.

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