Figure 1. The demo at work.

This demonstration explains the phenomenon of the “whispering galleries,” where one can speak softly at one place and is clearly heard in a remote location. There is one such gallery at the Exploratorium in San Francisco.

Materials:

• Dye & Water
• Elliptical Container
• Mechanical Vibrator

Demo:

The demonstration is based on an elliptical well containing water with a bit of dye. The two foci of the ellipse are marked. A disturbance at one focus from a drop of water or mechanical vibrator causes an identical disturbance at the other focus.

Explanation:

Because of the properties of an ellipse, waves emanating from one focus and reflecting from any point on the wall of the well arrive at the other focus in phase. This is an example of constructive interference. See the video in figure 2 for a simplified illustration of this concept.

Figure 2. From, Last Word on Nothing.

This demonstration can also be used for explanation of “quantum mirage.” This effect describes a spot where electron waves are focused so they reinforce each other producing a virtual image of the original atom.