A thin film of acetone (n=1.25) lies on the surface of a thick piece of water (n=1.30). White light shines down from air onto the thin layer of acetone. Destructive interference occurs for m=3 for a wavelength in air of 600 nm. What is the thickness of the thin film?

Explanation:

The wave of light is being reflected two times while going from lower to higher refractive index mediums

So there will be phase reversal two times .

Condition of destructive index

2 μ t = ( 2m+1) λ / 2

Substituting the values

2 x 1.25 x t = ( 2 x 3 + 1 ) x 600nm / 2

2.5 t = 2100 nm

t = 840 nm .

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