Solution to task 504 from Chertov´s for extramural stud

Solution to task 504 from Chertov´s for extramural stud

504. A convex side of a plano-convex lens is placed on a glass plate. From above, the lens is illuminated with monochromatic light with a wavelength of λ = 500 nm. Find the radius R of the lens if the radius of the fourth, dark Newton´s ring in reflected light is r4 = 2 mm.