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# lecture28 - Physics 2102 Jonathan Dowling Lecture 28...

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Lecture 28 Lecture 28 Physics 2102 Jonathan Dowling Ch. 35: Interference Ch. 35: Interference

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Interference: Example Interference: Example A red light beam with wavelength λ =0.625 μ m travels through glass (n=1.46) a distance of 1mm. A second beam, parallel to the first one and originally in phase with it, travels the same distance through sapphire (n=1.77). How many wavelengths are there of each beam inside the material? In glass, λ g =0.625 μ m/1.46= 0.428 μ m and N g =D/ λ g =2336.45 In sapphire, λ s =0.625 μ m/1.77= 0.353 μ m (UV!) and N s =D/ λ s =2832.86 What is the phase difference in the beams when they come out? The difference in wavelengths is N s -N g =496.41. Each wavelength is 360 o , so N=496.41 means ∆φ = Nx360 o =0.41x360 o =148 o How thick should the glass be so that the beams are exactly out of phase at the exit (destructive interference!) N=D/ λ s - D/ λ g = (D/ λ )(n 2 -n 1 )=0.31 (D/ λ )=m+1/2 A thickness D=(m+0.5) 2.02 μ m would make the waves OUT of phase. For example, 1.008 mm makes them in phase, and 1.010 mm makes them OUT of phase.
Thin Film Interference: The patterns of colors that one sees in oil slicks on water or in bubbles is produced by interference of the light bouncing off the two sides of the film. To understand this we need to discuss the phase changes that occur when a wave moves from one medium to the another where the speed is different. This can be understood with a mechanical analogy.

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Reflection, Refraction and Changes of Phase: Consider an UP pulse moving in a rope, that reaches a juncture with another rope of different density. A reflected pulse is generated. The reflected pulse is also UP if the
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## This note was uploaded on 11/18/2011 for the course PHYSICS 2102 taught by Professor Dowling during the Fall '10 term at LSU.

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lecture28 - Physics 2102 Jonathan Dowling Lecture 28...

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