Chapter 34, Problem 65GP

In a compact disc (CD), digital information is stored as a sequence of raised surfaces called “pits” and recessed surfaces called “lands.” Both pits and lands are highly reflective and are embedded in a thick plastic material with index of refraction n = 1.55 (Fig. 34-34). As a 780-nm wavelength (in air) laser scans across the pit-land sequence, the transition between a neighboring pit and land is sensed by- monitoring the intensity of reflected laser light from the CD. At the moment when half the width of the laser beam is reflected from the pit and the other half from the land, we want the two reflected halves of the beam to be 180° out of phase with each other. What should be the (minimum) height difference t between a pit and land? [When this light enters a detector, cancellation of the two out-of-phase halves of the beam produces a minimum detector output.]

FIGURE 34-34

Problem 65.