39. For complete destructive interference, we want the waves reflected from the front and back of the coating to differ in phase by an odd multiple of πrad. Each wave is incident on a medium of higher index of refraction from a medium of lower index, so both suffer phase changes of πrad on reflection. If Lis the thickness of the coating, the wave reflected from the back surface travels a distance 2Lfarther than the wave reflected from the front. The phase difference is 2L(2π
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