Sorry, you need a Java-enabled browser to see the simulation.
This java applet demonstrates standing electromagnetic waves in a 3-d rectangular box, or cavity resonator. To use the applet, double-click on one of the grid squares to select a mode. The TE modes are on top and the TM modes are on the bottom. Use the menu items to select the electric or magnetic field, and the way the field is displayed.

Full Directions.

Zip archive of this applet.

Waveguide applet.

More applets.

The source.