A flat-concave lens

A flat-concave lens with a 1.50 refractive index and a concavity of 8.00 m radius facing down is placed on a flat glass plate. With the lens perpendicularly illuminated from above by a light of wavelength 589 nm (in the air), an interference pattern appears with a central dark dot surrounded by rings, of which 50 are dark, including the outermost at the edge of the lens. This interference pattern appears due to the air film between the lens and the glass plate (as outlined in the figure). The thickness of the air layer at the center of the interference pattern and the focal length of the lens are, respectively,