An optical fiber uses flint glass clad with crown glass. What is the critical angle?
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OpenStax College Physics for AP® Courses, Chapter 25, Problem 22 (Problems & Exercises)

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This is College Physics Answers with Shaun Dychko. A fiber optic cable is consisting of flint glass— we'll call this index of refraction number one because this is where the light rays begin— and it is clad, which means it is surrounded by a layer of crown glass and so this is the second medium and it has an index of refraction we'll call n 2 and if we look up these indices of refraction table here, we have flint glass is 1.66 and crown glass is 1.52— that's from table [25.1]— and so n 1 is 1.66 in other words and n 2 is 1.52 and the critical angle, which is this incident angle with respect to the perpendicular to the interface such that the refracted ray has an angle of 90 degrees. This critical angle is the inverse sin of the second index of refraction divided by the first index of refraction. So that's the inverse sin of 1.52— index of refraction of crown glass— divided by 1.66—the index of refraction of flint glass— and this is 66.3 degrees.