A layer of transparent plastic (n=1.61) on glass (n=1.52) looks dark when viewed in reflected light whose wavelength is 589 nanometers in vacuum. Find the two smallest possible nonzero values for the thickness of the layer.
Answers are 183 nm and 366 but I don't understand it. I got 91.46 and 183. I realize I divided by two twice but why would you only divide by two once since it's destructive interference and you get 2 pathlengths from the standard equation.
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Sorry I got 91.46 and 274.379. You were supposed to add 1/2 wavelength for the second value but why can't you add 1 whole wavelength especially since destructive interference is 1/2 wavelengths, 3/2 wavelengths, 5/2...etc.?
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