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When a light passes from a denser to a rarer medium at critical angle of incidence the light rays graces through the surface of the denser medium.According to the law of reversibility of light same thing should happen when we reverse the direction of light.But, how can that be? How does the light know when to go into the denser medium?

Rajath Radhakrishnan
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    possible duplicate of Does light reflect if incident at exactly the critical angle? – John Rennie Sep 26 '13 at 17:38
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    The link I've posted is an exact duplicate of your question though you can be forgiven for not finding it as the title of the question isn't helpful. – John Rennie Sep 26 '13 at 17:39
  • There is NO law of reversibility of "light". In geometrical optics, which is a simplified mathematical model, the direction of rays is reversible, but in real physical world propagation of electromagnetic wave energy, there is no reversibility except in the trivial case of propagation in a single homogeneous medium. If any boundary, between media is involved, there is no reversibility. Fresnel reflection describes the ray splitting into reflected and transmitted beams. Reverse those two and you do not get one beam; you get a new fourth beam as well. –  Sep 27 '13 at 18:51
  • Time-reversal symmetry still holds over a boundary between media (though not if the media are lossy.) In the general case you would get four beams, yes, but if you reverse the reflected and transmitted beams and they come together again with the appropriate relative phase difference that they had in the forward case, then they will destructively interfere in the pathway of the fourth beam, and all the energy ends up in the original beam. – ptomato Sep 28 '13 at 05:06
  • That requires coherent light and also requires total optical paths less than the coherence length. The OP made no mention of coherent light. –  Sep 28 '13 at 20:46

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