Chapter 35, Problem 74P

GO Two rectangular glass plates (n = 1.60) are in contact along one edge and are separated along the opposite edge (Fig. 35-45). Light with a wavelength of 600 nm is incident perpendicularly onto the top plate. The air between the plates acts as a thin film. Nine dark fringes and eight bright fringes are observed from above the top plate. If the distance between the two plates along the separated edges is increased by 600 nm, how many dark fringes will there then be across the top plate?