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 left 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.
The photograph above shows the interference pattern.