Michelson Interferometer - Gas Cell


To show that the speed of light in air is different from the speed of light in a vacuum.◙


Michelson interferometer with laser, gas cell with hand pump.

Setup Time

10 min.




The gas cell is positioned in one arm of the interferometer between the beam splitter and the moveable mirror such that the surfaces of the cell are perpendicular to the laser beam. After obtaining a clear set of fringes, pump some of the air out of the cell. Measure the pressure before and after and the number of fringes passed while pumping. The refractive index of the gas varies directly with its density, and the index of refraction of a vacuum is one. Thus a graph of refractive index as a function of pressure must go through the point x=0, y=1. Determine the slope of the line by calculatin


Eric F. Cave and Louis V. Holroyd, Inexpensive Michelson Interferometer, AJP 23, 61, (1955). ◙T. G. Bullen, Notes on an Inexpensive Form of Michelson's Interferometer, AJP 27, 520-521, (1959). ◙G. Friedmann and R. Vidal, Modification to an Inexpensive Interferometer, AJP 30, 604, (1962). ◙Hans Weltin, Light Beats, AJP 30, 653-654, (1962). ◙Gary C. Babcock, The "Sagnac" Interferometer, AJP 30, 311, (1962). ◙Matt Bushner, Instantaneous Multiple Use of Michelson's Interferometer, TPT 15, 238-239, (1977). ◙Selective Experiments in Physics, The Michelson Interferometer, Cenco. ◙Palmer, Optics Exper