Between two glass plates with refractive indices n1 and n2 there is a thin layer of liquid. A ray of light propagating in the first plate at an angle i, passing through a layer of liquid, enters the second plate at an angle r (Fig. 133). Show that the angle of incidence i and the angle of refraction r obey the usual law of refraction sin i / sin r = n2 / ni\, regardless of the presence of a layer of liquid between the first and second plates. _

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