PURPOSE: An optical analog to the interference pattern from two identical sound sources.

DESCRIPTION: Small transparencies consisting of a series of equally-spaced concentric rings are superimposed on each other. The interference pattern from these two identical sources can be observed as the distance between the two sources is varied.

This demonstration seems to be temporarily missing, so in its place click PowerPoint Moire two-source demonstration. The first slide is simply a representation of a circular wave. The second slide consists of two identical circular wave patterns superposed on the PowerPoint slide. You may grab the outer pattern with your mouse and drag it around, showing various interference patterns. If you drag it horizontally across the PowerPoint slide then you may see: (1) a two-source pattern perpendicular to the line between the two individual sources, and (2) an interferometer type pattern along the extension of the line between the two individual sources.

This excellent model of two-source interference is used with much thanks with permission of James M. Coburn, Department of Physics, Utah State University, who has created it.

SUGGESTIONS: An excellent applet by Paul Falstad, illustrating two-source interference, can be viewed by clicking your mouse on its URL: http://www.falstad.com/ripple/. Select "two sources" from the menu and drag the sources to any desired position. Important parameters can be adjusted, such as source spacing and frequency.

REFERENCES: (PIRA 3B50.40) See also H2-22: INTERFERENCE - TRANSPARENCIES.

EQUIPMENT: Small plastic Klinger transparent slides, overhead projector.

SETUP TIME: None.