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Physics/Critical angle and total internal reflection


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:  There's a somewhat shorter article on it in wikipedia: and hyperphysics has a calculator for it if you just need numbers: 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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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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