A plano-convex glass lens of radius of curvature 1.8 m rests on an optically flat glass plate. Before the lens is placed on the plate a film of oil of refractive index 1.78 is deposited on the plate. The arrangement is illuminated from above with monochromatic light of 480-nm wavelength. The indexes of refraction of the lens and plate are 1.5. The radius r of a fringe is related to the thickness t of the film and the radius of curvature R of the lens through r = (2tR)^1/2. What are the radii of the first and second fringes? How do you determine t?