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Measurement of refractive index using a Michelson interferometer

The Michelson interferometer has long been a popular piece of equipment in the physics undergraduate laboratory. One possible application is the measurement of the refractive index of a thin parallel-side plate of transparent material of known thickness. (Alternatively, if the refractive index is known, the thickness of the plate may be determined.) This is accomplished by using 'white light' fringes superimposed on monochromatic fringes and is described in standard laboratory manuals (Whittle and Yarwood 1973). The author describes another method for measuring the refractive index using and monochromatic fringes and explains how plates ranging in thickness from a few micrometres to several millimetres may be measured.