PURPOSE: To demonstrate double slit interference using several calibrated pairs of slits.

DESCRIPTION: A slide containing four sets of double slits is positioned in the laser beam using a slide holder on a cross-carriage mount. Any of the four sets of slides can easily be slid into the beam. The four sets of slits have slit width a and separation d as follows:

a=0.04mm, d=0.25mm; a=0.04mm, d=0.50mm;

a=0.08mm, d=0.25mm; a=0.08mm, d=0.50mm.

If you use the edge of a piece of paper to cover up one of the slits, you can view the single slit diffraction pattern that modulates the double slit interference. Two-slit interference is the small-spaced light/dark pattern, and the single slit diffraction is the larger pattern of intensity increases and decreases. The central diffraction maximum (zero-order) contains about eleven interference maxima, and each first-order side diffraction maximum contains about four interference maxima.

SUGGESTIONS: See the web site How to Build a Two-Beam Interferometer, by Peter Monta, to build our own two-source interference experiment for only pennies.

See Question of the Week #65 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 6D10.10)

EQUIPMENT: Laser cart with fixed double slits.

SETUP TIME: None.