Interference Between Glass Plates
Demonstration of interference by a thin film.
Superpressure mercury lamp, glass plates, 20cm focal length convex lens, and light baffle.
Light reflecting from the rear surface of the front plate (left) and the front surface of the rear plate (right) interfere. When the reflected waves are in phase that color will be focused on a screen to the left of the picture by the large lens. When the reflecting waves are out of phase no reflected light is seen. The light baffle at the right prevents direct light from the source from getting into students' eyes. Flex the two glass plates to vary their spacing. A thin piece of paper or a hair can be squeezed between the plates at one end to produce a wedge filter.
G. P. Sastry and R. S. Saraswat, Localization of fringes by the pair of images method: Wedge fringes versus Newton's rings, AJP◙58, 595-598 (1990).