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Unformatted text preview: on the moon: W L D (240000 miles) 632 . 8 nm 1 cm 15 miles. 1 Ph12a Solutions Problem Set 9 December 1, 2009 Figure 1: Geometry of problem 9.4 9.30 The circular wave a distance L from the source will appear to be planar in a circular region of radius R if its phase is approximately constant across this region. The maximum phase dierence will be between an edge and center of the region as shown in Fig. 2. The path dierence is given by p L 2 + R 2-L L (1 + R 2 2 L 2 )-L = R 2 2 L . Hence the phase dierence is = 2 path dierence = R 2 L . Hence the phase is approximately constant over the plane of the circle to the extent that its area is small compared to L . 2 Ph12a Solutions Problem Set 9 December 1, 2009 Figure 2: Geometry of problem 9.30 . 3...
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