A laboratory dish, 30 cm in diameter, is half filled with a glucose/water soluti

Question

A laboratory dish, 30 cm in diameter, is half filled with a glucose/water solution. One at a time, 0.40?L drops of oil from a micropipette are dropped onto the surface of the solution, where they spread out into a uniform thin film. As more drops are added, the intensity of 620nm light reflected from the surface decreases. The intensity reaches a minimum after a total of 13 drops have been added. In all questions, assume that the oil's index of refraction is smaller than the glucose/water solution's index.

What is the index of refraction of the oil?

Explanation / Answer

Solution: For destructive interference due to thin films the condition is 2?d = ?/2, considering n = 0. We have to find the thickness due to a single drop here that can be calculated as h = V/A, where V and A are volume of liquid and area of the surface respectively. We get h = 5.71 nm. So, total thickness of 13 drops, d = 74.28 nm.

By calculating, we get ? = ?/4d = 2.09.

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