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Critical angle for glass is 42degree.Would this value change if a piece of glass is immersed in water?Explain. A similar doubt is  A ray of light is travelling from diamond to glass.Calculate the valueof criticalangle for the ray,if the refractive index of glass is 1.51 & that of diamond 2.47 . PLEASE PROVIDE ME SOLUTIONS TO THE ABOBE QUESTIONS & CLARIFY MY CONFUSIONS.
the critical angle is defined as the angle of incidence that provides an angle of refraction of 90degrees. Make particular note that the critical angle is an angle of incidence value. For the waterair boundary, the critical angle is 48.6degrees. For the crown glasswater boundary, the critical angle is 61.0degrees. The actual value of the critical angle is dependent upon the combination of materials present on each side of the boundary.
Let's consider two different media  creatively named medium i (incident medium) and medium r (refractive medium). The critical angle is the that gives a value of 90degrees. If this information is substituted into Snell's Law equation, a generic equation for predicting the critical angle can be derived. The derivation is shown below.
n_{i} *Â• sine() = n_{r} Â• sine ()n_{i} Â• sine() = n_{r} Â• sine (90 degrees)
n_{i} Â• sine() = n_{r}
sine() = n_{r}/n_{i}
= sine^{1} (n_{r}/n_{i}) = invsine (n_{r}/n_{i})
The critical angle can be calculated by taking the inversesine of the ratio of the indices of refraction. The ratio of n_{r}/n_{i}is a value less than 1.0. In fact, for the equation to even give a correct answer, the ratio of n_{r}/n_{i} must be less than 1.0. Since TIR only occurs if the refractive medium is less dense than the incident medium, the value of n_{i }must be greater than the value of n_{r}. If at any time the values for the numerator and denominator become accidentally switched, the critical angle value cannot be calculated. Mathematically, this would involve finding the inversesine of a number greater than 1.00  which is not possible. Physically, this would involve finding the critical angle for a situation in which the light is traveling from the less dense medium into the more dense medium  which again, is not possible.