A thin metal foil separates one end of two pieces of optically flat glass (n = 1.5), the gap between the plates is filled with water (n = 1.33), similar to Fig. 24-33. When a monochromatic light with 410 nm wavelenghth is incident normally, there are 37 bright fringes observed, and the last one is at the metal foil. What is the thickness of the foil?