Lecture 35: WED 15 APRLecture 35: WED 15 APRCh:33.8-10Ch:33.8-10Physics 2102Jonathan DowlingReflection and RefractionReflection and RefractionLaw of reflection (Light Bounces): theangle of incidence θ1 equals the angleof reflection θ’1. θ1 = θ’1Law of Refraction (Light Bends):!!n2sin!2= n1sin!1!!Snell's LawWhen light finds a surface separating two media (air andwater, for example), a beam gets reflected (bounces) andanother gets refracted (bends).n is the index of refraction of the medium. In vacuum, n!=!1. In air, n! ~!1. In all other media, n!>!1.Plastic GlassSpeed of Light is Slowed n>1v ! c / n!!!!&!!!!n " 1!!!!!#!!!!v $ cn=1n>1Hits SandRoadvLvRTurns LeftExampleExampleWater has n=1.33. How much does a beam incident at 45o refracts? n2 sin θ2= n1 sin θ1 sin θ2= (n1 /n2) sin θ1 =(1/1.33) sin 45o =0.0098θ2= 32oWater on Desert Road IllusionWater on Desert Road IllusionThe index of refraction decreases with temperature: thelight gets refracted and ends up bending upwards.We seem to see water on the road, but in fact we arelooking at the sky!Water on the Desert Road MirageChromatic DispersionChromatic DispersionThe index of refraction depends on the wavelength (color) of the light.The Single RainbowThe Double RainbowTotalInternalReflection!Total Internal ReflectionTotal Internal ReflectionFrom glass to air, the law of refraction uses n2<n1, so θ2> θ1: it mayreach 90o or more: the ray is “reflected” instead of “refracted”.For glass (fused quartz)n=1.46, and the criticalangle is 43o: optical fibers!Condition for TIR:n2 sinθ2= n1 sinθ1 &θ2 ≥ 90°n1>1n2~1θ1θ2The cut of the diamond favorstotal internal reflection. Mostrays entering the top of thediamond will internally reflectuntil they reach the top face ofthe diamond where they exit.This gives diamonds their brightsparkle.A fiber optic is a glass "hair" which is sothin that once light enters one end, it cannever strike the inside walls at less thanthe critical angle. The light undergoestotal internal reflection each time itstrikes the wall.Fiber optic cables are used to carrytelephone and computer communications.Fiber Optic CablesFIBER OPTIC GUIDED MISSILEPolarization By ReflectionPolarization By ReflectionDifferent polarization of light getreflected and refracted withdifferent amplitudes(“birefringence”).At one particular angle, theparallel polarization is NOTreflected at all!This is the “Brewster angle” θB,and θB!+ θr!=!90o.!!!="=###cos)90sin(sin221nnno12tannn=!"Polarizing SunglassesPolarized
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