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# Physics/Critical angle and total internal reflection

Question
When a ray of light travels from glass to air, then if the angle of incidence is gradually increased,the angle of refraction also increases but the intensity of the refracted ray keeps on decreasing.Why is it so?

I'm slightly confused by your message's title relative to your question.  When dealing with the critical angle, it becomes impossible for light to be refracted past 90 degrees (that would make it parallel to the surface), and total internal reflection takes over because refracted light rays no longer satisfy Snell's law.  This is not really a gradual process, however, it happens rather suddenly.  If you're talking about general reflection vs refraction, these are a good bit more complex.  The intensities for reflected and transmitted parts are given by Fresnel's equations, and derived in some detail here: http://www.tcd.ie/Physics/people/Charles.Patterson/teaching/PY4P05/Fresnel_Equat  There's a somewhat shorter article on it in wikipedia: https://en.wikipedia.org/wiki/Fresnel_equations and hyperphysics has a calculator for it if you just need numbers: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/freseq.html Detailed derivation of Fresnel's equations themselves involves (as you can see) quite a bit of wave mechanics and detailed application of Maxwell's equations, somewhat beyond the scope of this forum (really more appropriate for a graduate electrodynamics course).

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