haha my student asked the same question... She's not online, if
not I will get her to explain. :x
Basically, it is not 0.5 wavelength path difference. You
have to take into account that when the distance from one end
reduces by 0.5 wavelength, the distance from the other end
increases by 0.5 wavelength.
Hence in total, the path difference would have increased by 1
full wavelength, and not 0.5 wavelength.
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