Written by W. Christian at Davidson College.

*Additional comments by J. Varriano:*

**Classical**

The applet shows a moving object emitting sound waves.

The number in the box on the right shows the *Mach number * which is the ratio of the speed of the object to the speed of sound.

**Relativistic**

The applet shows a moving object emitting light waves.

The number in the box on the right shows the ratio of the speed of the object to the speed of light.