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# Assign10 - PHY 54 Assignment 10 Summer 2011 Reading Wave...

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Reading : Wave Optics 1, 2 Key concepts: Superposition; phase difference; amplitude and intensity; thin Flm interference; ±raunhofer diffraction; gratings; resolving power. 1. Questions about interference of waves. a. Why do we not see interference patterns in the light from the two headlights of a distant car? b. You are standing outdoors near a corner of a building. Two people around the corner are talking. You can hear them but not see them. Why? c. You know the two people are talking, but you are unable to understand their conversation. Why? [They are speaking your language.] 2. Questions about thin Flms. a. When a Flm of water on glass evaporates the re±ected light becomes very bright just before the Flm is gone. Why? b. When a soap Flm in air gets thin just before breaking the re±ected light is very dim. Why? c. A circular Flm of oil on water is thinnest near the edge. In the re±ected light, would one see blue or red colors in that region of the Flm? 3. In Assignment 8 you examined the intensities of the re±ected and transmitted rays shown. To analyze the interference of the waves you must know the amplitudes also. The relations between amplitudes upon re±ection are: A refl = R A inc , A trans = T A inc . a. Show that if R << 1 then we have A b A a >> A c . [This is why we considered only waves a and b in analyzing interference in the re±ected light from a thin Flm.] b. Show that in this case A a A 0 >> A b >> A c . [This is why interference effects in the transmitted light are negligible for ordinary Flms.] c. Show that if R 1 we have A a A 0 >> A b >> A c , so usually nearly all the light is re±ected and interference is negligible. a b c A 0 a b c PHY 54 Assignment 10 Summer 2011

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4. The exception to the conclusion in 3(c) occurs when all the transmitted waves interfere constructively. a.
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## This note was uploaded on 10/19/2011 for the course PHYSICS 54L taught by Professor Thomas during the Summer '09 term at Duke.

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Assign10 - PHY 54 Assignment 10 Summer 2011 Reading Wave...

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