49. Consider the interference pattern formed by waves reﬂected from the upper and lower surfaces of the air wedge. The wave reﬂected from the lower surface undergoes a π rad phase change while the wave reﬂected from the upper surface does not. At a place where the thickness of the wedge is d , the condition for a maximum in intensity is 2 d =( m + 1 2 ) λ ,whe re λ is the wavelength in air and m is an integer. Thus, d =(2 m +1) λ/ 4. As the geometry of Fig. 36-34 shows, d = R − √ R 2 − r 2 ,where R
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