Underwater Interferometric Seismometer

Overview

Objectives/Goals In recent years, due to the great strides in nanotechnology and other fields, it has become a priority to measure microscopic distances. This project was designed to enhance the effectiveness of an interferometer, a device used to measure such distances, with a simple and inexpensive method. Methods/Materials A Michelson interferometer was assembled using a 532 nm wavelength green laser, a steel frame, two 50/50 cube beamsplitters, two 25 mm radius mirrors, a 25 mm square mirror, two 22 mm diverging lenses with a -15 mm focal length, and a large reflector. A measurement system was assembled using two photodetectors, two amplifiers, and a data acquisition unit. For each test one mirror was submerged in a liquid, and one mirror was left above the liquid to act as a control. The amount of light shining on the photodetectors was recorded in 0.5 second intervals for each test. This would record any lines of darkness called #fringes# that pass over the photodetectors. The first test was a control to determine if the 2 mirrors had the same sensitivity. The following 5 tests involved submerging the test mirror in 5 different liquids. The data was saved in a spreadsheet format and copied to a computer where it was put through a series of calculations to determine the average amount of time it takes each fringe to pass over the photodetector. Results After the data was adjusted for the 12.7% difference in the control test, the first test where the test mirror was submerged in water, there was a 6.9% increase from the reference mirror and the test mirror. In the second test (Karo syrup), there was a 15.8% increase from the reference mirror and the test mirror. In the third test (mineral oil), there was a 18.6% increase from the reference mirror to the test mirror. In the fourth test (olive oil), there was a 9.5% increase from the reference mirror to the test mirror. In the fifth test (methyl alcohol), there was a 27.9% increase from the reference mirror to the test mirror. Conclusions/Discussion Overall, the data did support the hypothesis. Submerging the reference mass of an interferometer in liquid showed up to a 28% increase in sensitivity. Some sources of error that may have altered the results of this experiment include vibrations that were not in the center of gravity of the 2 mirrors and the lid used to halt vibrations on the surface of the liquid may have bumped the mirrors and caused them to vibrate.

Competition history

• CSEF 2008 Physics & Astronomy · Entry S1602

Resources

• CSEF project summary ↗

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