Physics 2102 Jonathan Dowling Lecture 35 WED 15 APR Ch 33 8 10 Reflection and Refraction When light finds a surface separating two media air and water for example a beam gets reflected bounces and another gets refracted bends Law of reflection Light Bounces the angle of incidence 1 equals the angle of reflection 1 1 1 Law of Refraction Light Bends n2 sin 2 n1 sin 1 Snell s Law n is the index of refraction of the medium In vacuum n 1 In air n 1 In all other media n 1 Plastic Glass Speed of Light is Slowed n 1 n 1 n 1 Hits Sand Turns Left Road v c n n 1 v c vL vR Example Water has n 1 33 How much does a beam incident at 45o refracts n2 sin 2 n1 sin 1 sin 2 n1 n2 sin 1 1 1 33 sin 45o 0 0098 2 32o Water on Desert Road Illusion The index of refraction decreases with temperature the light gets refracted and ends up bending upwards We seem to see water on the road but in fact we are looking at the sky Water on the Desert Road Mirage Chromatic Dispersion The index of refraction depends on the wavelength color of the light The Single Rainbow The Double Rainbow Total Internal Reflection Total Internal Reflection From glass to air the law of refraction uses n2 n1 so 2 1 it may reach 90o or more the ray is reflected instead of refracted 2 Condition for TIR n2 sin 2 n1 sin 1 2 90 n2 1 1 n1 1 For glass fused quartz n 1 46 and the critical angle is 43o optical fibers The cut of the diamond favors total internal reflection Most rays entering the top of the diamond will internally reflect until they reach the top face of the diamond where they exit This gives diamonds their bright sparkle A fiber optic is a glass hair which is so thin that once light enters one end it can never strike the inside walls at less than the critical angle The light undergoes total internal reflection each time it strikes the wall Fiber optic cables are used to carry telephone and computer communications Fiber Optic Cables FIBER OPTIC GUIDED MISSILE Polarization By Reflection Different polarization of light get reflected and refracted with different amplitudes birefringence At one particular angle the parallel polarization is NOT reflected at all This is the Brewster angle B and B r 90o o n1 sin n2 sin 90 n2 cos n2 tan n1 Polarizing Sunglasses Polarized Sunglasses
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