Newton's rings by transmission or reflection on arc bench.

In the Newton's Rings Apparatus, a piece of flat glass is sandwiched against a piece of glass that has a slight positive curvature (a long focus plano-convex lens). The interference fringe pattern produced in the air film between the glass pieces is concentric rings. Transmission Case: Collimated white light from the carbon arc passes through the Newton's Rings Device and is focused on a projection screen by lens 1. The center of the fringe ring pattern is bright. Reflection Case: Collimated white light from the carbon arc reflects off of the Newton's Rings Device and reflects again from a front surface mirror, then is focused on a projection screen by lens 2. The central spot (where the two glass surfaces touch) is dark. There, light reflected from the flat surface changes phase by 180, whereas light internally reflected from the curved surface does not. Newton's rings by transmission or reflection on arc bench. Ref.: Modern College Physics by Harvey White,6th Ed., p.546-548