A set up for the Twyman-Green interferometer is shown in (Fig.A(a)). The interferometer is used to test the flat surfaces, beam splitters, prism and lens. Light from a source S is collimated using lens L1 which falls on a beam splitter and is divided into two beams which move towards mirrors M1 & M2 respectively. These beams are then reflected back and combine again at beam splitter. Another lens L2 is used to project the fringe pattern on screen. The mirrors and beam splitters used in the set up are of high optical quality. Interference phenomenon takes place between two plane