Looking straight downward into a rain puddle whose surface is covered with a thi

Question

Looking straight downward into a rain puddle whose surface is covered with a thin film of gasoline, you notice a swirling pattern of colors caused by interference inside the gasoline film. The point directly beneath you is colored a beautiful iridescent green. You happen to remember that the index of refraction of gasoline is 1.39 and that the wavelength of green light is about 539 .

Explanation / Answer

the path difference is 2*n*d (where d is the thickness of the gasoline at that point) because n_gas=1.39>n_water=1.33 so the phase diffence is added by an amount of pi/2 so path difference (phase). 2*n*d+lamda/2. constructive interference with lamda=539. so 2*1.39*d+539/2=539 so d=97(nm)

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