Compact Disc Player a compact disc (CD) is read from the bottom by a semiconductor laser with wavelength 790nm passing through a plastic substrate of refractive index 1.8. When the beam encounters a pit, part of the beam is reflected from the pit and part from the flat region between the pits, so these two beams interfere with each other (Fig. 3522). What must the minimum pit depth be so that the part of the beam reflected from a pit cancels the part of the beam reflected from the flat region? (It is this cancellation that allows the player to recognize the beginning and end of a pit. For a fuller explanation of the physics behind CD technology, see the article ''The Compact Disc Digital Audio System," by Thomas D. Rossing, in the December 1987 issue of The Physics Teacher.)

Plastic substrate Pits Reflective coating Laser beam