Michelson-interferometer (0725) In a Michelson-interferometer a coherent light beam is separated into two rays travelling in different directions by inserting a half-silvered mirror in the path. The separated beams are reflected back along themselves and then recombined. The viewing scree n then exhibits an interference pattern that changes perceptibly when the optical path lengths for each of the split beams are changed by fractions of the wavelength of light. EXPERIMENT PROCE DURE : • Determine the wavelength of a laser. • Determine the refractive index of air as a function of pressure. • Determine the refractive index of glass. • Determine the quality of the surface of a strip of adhesive tape. BASIC PRINCIPLE : The Michelson interferometer was invented by A. A. Michelson originally to demonstrate whether the Earth could be observed to be in motion through an ether in which light was once thought to propagate. His design has nevertheless proved crucial for making interferometric measurements, e.g. of changes in length, thickness of layers or refractive indices. A divergent light beam is split into two by a half-silvered mirror and the two resulting beams travel along differing paths. They are then reflected back on themselves and recombined so that interference patterns can be viewed on a screen. The resulting pattern is highly sensitive to any differences in the optical paths covered by the split beams. If the refractive index remains constant the degree of change in the geometric paths can be calculated, e.g. changes in size of various materials due to thermal expansion. If by contrast the geometry is maintained, then refractive indices or changes in them due to pressure, temperature or density variations may be determined. Qty :