The perfectly flat piece of glass (n = 1.50) is placed over a perfectly flat pie

Question

The perfectly flat piece of glass (n = 1.50) is placed over a perfectly flat piece of plastic (n = 1.20) as shown in the figure below. They touch at A. Light of wavelength 600 nm in air is incident normally from above. The location of the dark fringes in the reflected light is shown on the sketch of the figure below. (a) How thick is the air film between the glass and the plastic at B? (b) Water (n = 1.33) seeps into the region between the glass and the plastic. How many dark fringes are seen when all the air has been displaced by wear? (The straightness and equal spacing of the fringes is an accurate test of the flatness of the glass.)

Explanation / Answer

phase change occurs only when n2 > n1

Since water is denser than plastic no phase change occurs

So destructive interference happens at the edge

2t = m (lambda)

I dont find anything wrong in answer except destructive occurs in the start and m =8

So total dark fringes will be 9

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