Hmm, more questions on phase change. I was looking at this question: "Waves from a radio station have a wavelength of 319 m. They travel by two paths to a home receiver 14.2 km from the transmitter. One path is a direct path, and the other is by reflection from a mountain directly behind the home receiver. What is the minimum distance from the mountain to the receiver that produces destructive interference at the receiver? (Assume that no phase change occurs on reflection from the mountain.) Answer: 7.98e+01 m" Now, it looks like the answer, to me, implies that the path difference is lambda/4. Why would it be lambda/4 and not lambda/2? Lambda/2 gives complete destructive interference.