# lecture33 - Diffraction Ch 38 Interference effects for...

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Interference effects for continuous sources: i) Light bends around corners. i) “Shadows” fill in i) “Parallel” beams always spread i) Limits of resolution of microscopes and telescopes Diffraction Ch 38 Diffraction Ch 38

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Fraunhofer Diffraction : (easy math) Source, screen “ at infinity”, ~parallel rays eg. Laser & narrow slit. Fresnel Diffraction : (complicated math) Source distance, object size and screen distance all comparable. eg. Source Steel Ball Screen Shadow Poisson Spot
SingleSlit, width = SingleSlit, width = a a a θ 5 4 3 2 1 Use Huygens’s idea: Treat the slit as a largenumber of point sources. x = a sin θ is path difference between waves at two edges (Fraunhoffer diffraction)

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Divide slit in half and add up rays in pairs : Ray (1) travels farther than ray (3): when it is ½ cycle behind -> Cancel Ray (2) travels farther than ray (4): when it is ½ cycle behind -> Cancel Ray (3) travels farther than ray (5): when it is ½ cycle behind -> Cancel 1) ANY two waves that originate at points a/2 away are out of phase by 180 o
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