🌈 2D Newton's Rings: Lens on an Optical Flat, Radius of Curvature and Wavelength

Newton's rings from a plano-convex lens on an optical flat: gap thickness d = r^2/2R, dark ring radii sqrt(m lambda R / n), monochrome and white-light colours from the CIE colour matching functions, liquid in the gap, dust at the contact point and a two-ring measurement of R.