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This experiment teaches step-by-step procedures for setting up a Mach Zhender interferometer, a Sagnac Interferometer, and a Michelson interferometer. To obtain clear concentric interference patterns at the output, the wavefront of the optical wave in one interferometer arm is deliberately adjusted differently from that in the other interferometer arm. A Mach Zhender interferometer is used to demonstrate complimentary interferometer patterns at its two output ports. As a common-path interferometer, a Sagnac interferometer is used to illustrate a much stable interference fringes compared with those from the other two interferometers. In the end, a glass plate is inserted into a Michelson interferometer and a Mach Zhender interferometer for precise refractive-index measurements.