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**Unformatted text preview: **Class 16 Total Internal Reflection and Huygens’ Principle Physics 106 Fall 2011 Press CTRL-L to view as a slide show. Learning Outcomes Last time we discussed: I What you should know about electromagnetic radiation I What is light? I Reflection I Refraction and Snell’s Law I Total Internal Reflection Learning Outcomes Today we will discuss: I Total Internal Reflection I Huygens’ Principle I Terminology I Spherical Mirrors, Ray Diagrams, and the Mirror Equation Total Internal Reflection Total Internal Reflection Total Internal Reflection I Total internal reflectioncan only occur when light moves from a region of higher index of toward a region of lower index of refraction Total Internal Reflection I As light refracts, it bends away from the normal. I As the angle of incidence increases, the angle of refraction eventually reaches 90 ◦ Total Internal Reflection I When θ 2 = 90 ◦ , the angle of incidence is called the critical angle, θ c I No light leaves the more dense medium for θ 1 > θ c Critical Angle I The critical angle satisfies Snell’s law with θ 2 = 90 ◦ sin θ c = n 2 n 1 for n 1 > n 2 Fiber Optics I Plastic or glass rods use total internal reflection to "pipe" light from one place to another I Applications include I medical use of fiber optic cables for diagnosis and correction of medical problems I telecommunications Huygens’ Principle Christiaan Huygens I 1629 -1695 I Best known for contributions to fields of optics and dynamics I Deduced the laws of reflection and refraction I Explained double refraction Huygens’ Principle I Huygens assumed that light is a form of pressure I Pressure from a point source transfers pressure to adjacent points, producing a spherical wave front. Huygens’ Principle I For our purposes, Huygens’ Principle is a geometric construction for determining the position of a new wave at some point based on the knowledge of the wave front that preceded it Huygens’ Principle I All points on a given wave front are taken as point sources for the production of spherical secondary waves, called wavelets I After some time has elapsed, the new position of the wave front is the surface tangent to the wavelets Huygens’ Construction for a Plane Wave I At t = 0, the wave front is indicated by the plane AA I The points are representative sources for the wavelets...

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