Flat Glass Plates are used for demonstrating 'Newton rings'. These are patterns formed as light passes from a dense medium, through an extremely thin layer of air and back into a dense medium again. Two strips of flat glass are held apart at one end by a thickness of very thin paper and the glass strips are permitted to touch at the other end. This creates a 'wedge' of air between the plates. Using a monochromatic light source (a sodium source is normal), the interference patterns are easily seen.  Instructions included.