lec20(2x2) - LECTURE 20 Interference Diffraction...

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LECTURE 20: Interference, Diffraction, Resolution !""#$%%&’"()#*+,-.*’&,&*+/0%&#/1%23&+4%5657%87"9.:.!&’1)&:-*;#+ 11/6/11 3 Interference *Three ways in which the phase difference between two waves can change: 1. By traveling though media of different indexes of refraction 2. By traveling along paths of different lengths 3. By reflection from a boundary A wave will undergo a 0.5 wavelength phase shift if it reflects off a medium with higher index of refraction 11/6/11 4 Interference: Double Slit (for very narrow slits) *The phase difference between two waves can change if the waves travel paths of different lengths. Interference maxima for d sin ! m = m " , m = 0,1,2,... Interference minima for d sin ! m = m + 1 2 # $ % & ( " , m = 0,1,2,... Flaring of light
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11/6/11 5 Interference: Double Slit ( DEMO ) two slits Laser ( L ight A mplification by S timulated E mission of R adiation) He, Ne laser Interference patterns visible on screen (Coherent light source) 11/6/11 6 Diffraction * Recall that when a wave encounters an edge, obstacle, or aperture with a size much bigger than its wavelength ! ( d<< ! ) , light travels in a straight line as rays and a sharp bright image of the slit is formed on the screen. This regime is called geometrical optics . * However, when a wave encounters an edge, obstacle, or aperture with a size comparable to its wavelength ! ( d "! ) , those waves spread out. This effect is called diffraction. This regime is called wave optics (geometrical optics is no longer a good approximation).
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