Thin films: A thin layer of oil (n = 1.25) is on top of a puddle of water (n = 1.33). If normally incident 500-nm light is strongly reflected, what is the minimum nonzero thickness of the oil layer?
A) 200 nm
B) 250 nm
C) 100 nm
D) 400 nm
E) 150 nm

Question

Quizplus · Accepted Answer

For strong reflection, the path length difference between two successive reflected waves must be an odd multiple of half a wavelength. The path length difference is 2nt, where n is the index of refraction of the oil layer and t is its thickness. For 500-nm light, the path length difference must be 250 nm. Therefore, the minimum nonzero thickness of the oil layer is 250 nm / 2 = 125 nm.

Thin Films: a Thin Layer of Oil (N = 1