Revisiting Fizeaus Observations: Spectral study of Na source using Newtons rings

Conventional Newtons rings is generally used in undergraduate laboratory to determine the average wavelength of Na doublet. A minor modification of this setup enables us to view simultaneously Newtons rings both in reflected as well as transmitted light. A movable glass plate with respect to the plano-convex lens allows us to observe the variation of contrast/visibility of these fringes and thus allows us to determine the separation of the yellow doublet.