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“Ray Optics: Reflection, Refraction, Polarization”
Physics 117B02 — March 19 “Ray Optics: Reflection, Refraction, Polarization” 9/19/2018 PHYS117B.02 JV
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Previous 2 lectures: Chapter 32
Major theme: electromagnetic waves Predicted by Maxwell, verified by Hertz Radiation from accelerated charges (e.g., dipoles) Harmonic (sinusoidal) disturbance of E and B fields Travel with speed c in vacuum, c/n in matter in vacuum, in matter EM waves transport energy and momentum Poynting vector gives power/unit area Applications Communications, solar heating, laser cutting, welding, surgery Radiation pressure (e.g., solar “wind”) 9/19/2018 PHYS117B.02 JV
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Today’s lecture: Chapter 33
Major themes: reflection, refraction, polarization Rays and wavefronts: Huygens’ principle Reflection and refraction from interfaces Dispersion and total internal reflection (rainbows) Polarization effects due to reflection and scattering Scattering Applications Why objects look shifted under water Telecommunication through optical fibers Medical endoscopes and fiberscopes What causes rainbows How do polarization glasses work How to make 3d movies using polarization of light Why is the sky blue and the sunset - red 9/19/2018 PHYS117B.02 JV
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Terminology Light rays travel perpendicular to phase fronts
Each point on wave front acts as source of secondary spherical or circular “wavelets” … and Phase fronts are the tangents to those spheres or circles (Huygens’ principle) 9/19/2018 PHYS117B.02 JV
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Limits of ray optics When observing EM waves at great distances from their source, it is useful to assume that light travels in a straight line (perpendicular to the wavefronts) unless it encounters an obstacle. 9/19/2018 PHYS117B.02 JV
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Specular vs diffuse Light incident on smooth reflecting surfaces is reflected at a specular angle; if the surface is rough, the light scatters in many directions from the asperities in the surface, and there is both specular and diffuse reflection. 9/19/2018 PHYS117B.02 JV
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Fermat’s principle: reflection
Fermat’s idea was that light follows the geometrical path that minimizes the transit time. Consider the case of reflection from a surface in air. The total transit time from point A to point B is The minimum travel time is found by taking the derivative with respect to y and setting it equal to zero: 9/19/2018 Problem 33.52 PHYS117B.02 JV
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The critical angle for TIR
Consider light incident from glass (n=1.5) to air (n=1.0) at several different angles (see figure). Snell’s law relates the angle of incidence to the angle of refraction. J1 J2 0˚ 30˚ 40˚ 45˚ ??? At the critical angle … 9/19/2018 PHYS117B.02 JV
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Application of TIR: prisms
Total internal reflection in glass or crystalline quartz prisms are frequently used in optical systems instead of mirrors. The prisms below all are cut at angles of 45˚ and 90˚. Why? EXAMPLES Cameras Periscopes High-power laser beams Can you think of others? 9/19/2018 PHYS117B.02 JV
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Optical fibers Why communicate with light? (in a word, bandwidth!)
Added advantage: not subject to electrical interference Fiber transmission works because of total internal reflection at core-cladding interface Fibers can support multiple transmission paths (multi-mode) or only one (single-mode) 9/19/2018 PHYS117B.02 JV
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Polarization: Polarizers work by preferred direction of absorption
Malus’s law for polarized light passing through a polarizer-analyzer pair Polarizers work by preferred direction of absorption Long, stringy molecules work for visible light If you have owned a pair of polarizing sunglasses … Have you noticed a change in brightness of sky when you put them on? What does this mean? 9/19/2018 PHYS117B.02 JV
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Polarization by reflection
The Brewster angle … Reflections can polarize light that was initially not polarized … (Brewster’s angle) Since scattering is a form of reflection, what happens to polarized light scattered from a rough surface? Then why is it smart to wear polarizing sunglasses when driving in bright sunlight? 9/19/2018 PHYS117B.02 JV
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Summing up … Light rays travel in straight lines until they run into something Even when they bump into something, light rays travel the path that takes the shortest time In material media (e.g., glass) the speed of light depends on its color (dispersion) When light is reflected from, scattered from or transmitted through a surface, it may be polarized The degree of polarization caused by reflection depends on the angle of incidence 9/19/2018 PHYS117B.02 JV
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