Find the height h of the pits on a CD (Fig. 25.6a). When the laser beam reflects partly from a pit and partly from land (the flat aluminum surface) on either side of the "pit," the two reflected beams interfere destructively; h is chosen to be the smallest possible height that causes destructive interference. The wavelength of the laser is 780 nm and the index of refraction of the polycarbonate plastic is n = 1.55.

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Label Acrylic Polycarbonate plastic Aluminum (reflective layer)