We discuss Young's double-slit experiment using a partially coherent light source consisting of a helium-neon laser incident on a rotating piece of white paper. Such an experiment is appropriate for undergraduate students as an independent project or as part of an advanced lab course. As is well known, the resulting interference pattern is observed to disappear and return, depending on the angular size of the source. Interestingly, while the standard theoretical prediction for the light intensity agrees quite well with experimental data when the fringe visibility is high, the prediction is noticeably off when the visibility is low. A first-principles calculation of the light intensity is performed and shown to agree extremely well with the experimental results for all visibilities.

1.
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For coherent sources one adds the fields before squaring to get the intensity. For incoherent sources the random phase relationship means the “interference term” time-averages to zero. The result is a superposition of intensities for incoherent sources.

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18.
The Altera DE2 is a commercially produced development and education board. Users must load a program onto the board in order to interface with LabVIEW. We used a coincidence-counting program developed by Mark Beck for single-photon quantum mechanics experiments. For additional information, see Ref. 16 and/or Mark Beck's webpage <http://people.whitman.edu/∼beckmk/QM/>.
19.

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The sine integral function is one of the so-called trigonometric integrals, all similarly defined; see <https://en.wikipedia.org/wiki/Trigonometric_integral> for additional information.
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© 2018 American Association of Physics Teachers.
2018
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