The yellow lines of sodium are split due to spin-orbit coupling.  This line split is measured three ways in this experiment:  first using a grating spectroscope, then visually on a Fabry-Perot interferometer, and finally using the Fabry-Perot interferometer with output to a photodiode connected to a chart recorder.   The student will already be familiar with the use of the grating spectroscope from the H-alpha lab.  Using the values from the H-alpha lab for the separation of the rulings on the reflection grating, the student can easily find the change in wavelength between the two yellow sodium lines.   Next the student sets up the sodium source in front of a Fabry-Perot interferometer.  Using their eye as the lens, the student can see that the split yellow lines resolve themselves into separate lines, and then overlap as the separation of the mirrors is changed.  By counting fringes that pass between overlapped and resolved stages, the student can measure the line split.  Finally, the student sets up the interferometer so that the central fringe is imaged on a photodiode.  Then, while changing the mirror separation at a constant rate, the student makes a chart recording.  By counting fringes between resolved and unresolved spectral lines on the chart recording, the student can again measure the line split.

This page was prepared by Barbara Mattson (Fall '97), modified by K.M.

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