Chapter 25.6, Problem 2aTH

A plate of glass $\left(n=1.5\right)$ is placed over a flat plate of plastic $\left(n=1.2\right)$ . A thin film of water $\left(n=1.33\right)$ is trapped between the plates as illustrated in the diagram at right.

Light of frequency $f=4.8\times {10}^{14}Hz$ is incident from above at essentially normal incidence. The locations of the dark and bright fringes, as viewed from above, are indicated in the top view diagram at right.

In this problem, ignore reflections from the top surface of the glass plate and the bottom surface of the plastic.

1. Are the plates in contact at point A (where the plate appears dark) or at point B (where the plate appears bright)? Explain your reasoning.