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INDIANA UNIVERSITY, DEPARTMENT OF PHYSICS, P309 LABORATORY Laboratory #40: Michelson Interferometer, The Refractive Index of a Gas Goal: Measure the refractive index of gases using a Michelson interferometer Equipment: Michelson interferometer, gas cell, compensator, absolute pressure gauge, cylinder with helium gas, vacuum pump. 1. Experimental setup: The Michelson interferometer is illustrated in Fig. 1. A bundle of light from the light source falls on the beam splitter, which on its rear surface has a reflecting coating. The beam splitter divides the light into two beams, one transmitted toward the fixed mirror M2, the other reflected toward the moveable mirror M1. Both mirrors return the light to the reflecting surface where the two rays recombine and proceed toward the observer. The recombined rays interfere with each other, which results in bright and dark regions in the field of view. A change of half a wavelength in either of the two legs will be a visible effect. Figure 1: Michelson Interferometer The interferometer must be aligned before it can be used. This is achieved by means of the two adjustment screws that allow you to tilt the mount of mirror M2, and with the main micrometer screw which moves mirror M1 a directly measured distance perpendicular to its plane (there is a 5:1 gain caused by the lever). First you have to set the moveable arm such that the path length in the two legs is the same. This is done by measuring with a ruler the distance from the center of the beamsplitter to the center of each mirror. The second step is to align the mirrors M1 and M2 exactly perpendicular to one another. Illuminate the beamsplitter with a quasi-monochromatic source. In our case, this is a mercury light source, diffused by a ground glass screen, followed by an interference filter that passes only the green λ = 546.1 nm line. To help with the alignment, a pin is mounted between the light source and the beam splitter. When the two mirrors are misaligned, two images of the pin appear in the field of view. Adjust the screws on mirror M2 untill the two
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