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The Michelson Interferometer
This lab involves a couple different experiments, which
utilize a Michelson Interferometer to measure the wavelength of a Helium/Neon
Laser and the index of refraction of air. The Michelson Interferometer
is the most fundamental form of a number of two-way interferometer configurations
that are used to measure various optic properties or even distances and
thicknesses.
The equipment used here consists mainly of a Michelson Interferometer. The setup of such a device, shown above, requires a laser and a series of mirrors and lenses, along with a beam splitter and a compensator plate. The laser emits its light, which passes through the beam splitter. Two different beams of light leave the beam splitter, with half of the original beam going toward each of the mirrors in the setup. The mirrors reflect the light back to the beam splitter, where both beams are partially transmitted to the viewing screen. The lens in the beginning of the laser path helps spread out the beam, which allows fringes to be seen when the beam finally hits the viewing screen. A. A. Michelson deisgned his apparatus based upon the same princples used by Thomas Young. Light can be modeled as a wave of oscillating electric and magnetic fields. These fields add upon eahc other when different beams of light meet, producing the vector sum of the waves as the resulting light emission. Oscillations in light intensity result from this. Two beams of light from different sources generally have no fixed relationship in their oscillations, as their phases are unrelated. With such variable oscillations occuring within very short time periods, the human eye just averages the intensities seen in the light and keeps that as a constant. If the two light beams originate from the same source, however, their close phase and frequency relationship is close enough that there are consistent places of maximums and minimums of light intensity. The human eye can observe these as bright spots and dark spots. The first experiment in this lab aims to determine the wavelength of the laser source being used. The moveable mirror is adjusted, and the number of fringes that occur on the viewing screen is counted. A predetermined number of fringes are counted, which must be higher than 20, and then the distance travelled by the moveable mirror during the experiment is measured. Using this, the wavelength of the laser can be calculated. The procedure is repeated with a photometer and fiber optic probe used in place of the viewing screen, as well. Laser wavelength calculations are done in each case, and error analysis is done at the end using the true value of the laser's wavelength. The second experiment in interferometry conducted determines the index of refraction of air. The wavelength of light is determined by its wavelength in vacuum and the index of refraction of the medium it is currently moving through. For gas at lower pressures, the index of refraction varies linearly with with gas pressure, which allows an experiment to be done that combines known gas pressures with an experimentally determined slope for a graph of index of refraction vs. gas pressure. A vacuum cell is placed between the beam splitter and the moveable mirror and a starting pressure reading is taken from the cell pressure readout. The air in the vacuum cell is slowly pumped out as fringes are counted as they appear. When the vacuum has completed pumping air out of the cell, a final pressure reading is taken and the number of fringes counted is recorded. These two things allow for a slope of pressure vs. index of refraction to be calculated. Applications of Michelson Interferometry are extensive. European meteorological satellites utilize interferometry to interpret atmospheric activity and weather patterns, as well as temperature transients. Optical communication is another application, and is a continually developing field that offers incredible things to the future. Spacecraft, satellites, and other space applications will necessitate the accuracy and speed of long-distance communication offered by optical methods. Optical communication offers faster, almost instantaneous, communication here on Earth as well, which could be utilized for many different purposes including the Internet and military communications. |
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NEEP Lab Course Instructor:
Yaron Danon, vist Dr.
Danon's web site |
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