Light from a low-cost He–Ne laser is filtered to provide a single-mode beam of long coherence length. Beat frequencies in the 100- to 500-kHz range are produced by superimposing laser radiation and waves which are Doppler shifted by a moving retroreflector. Varying speeds of a model train mounted reflector are independently measured with a photogate and allow a check of the Doppler predicted speed, or the photogate determined speed can be used to measure the laser wavelength by a fit of data points with a resulting standard deviation of less than 0.5% of λ. A frequency to voltage converter allows direct velocity plots for constant acceleration or simple harmonic motion.
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© 1997 American Association of Physics Teachers.
1997
American Association of Physics Teachers
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