3.5 Exercise 30

Two identical pieces of rectangular plate glass are used to measure the thickness of a hair. The glass plates are in direct contact at one edge and a single hair is placed between them hear the opposite edge. When illuminated with a sodium lamp ($\lambda = 589 \text{nm}$), the hair is seen between the 180th and 181st dark fringes. What are the lower and upper limits on the hair’s diameter?