is a demonstration of Aliasing.
When the shape of a waveform is gathered and stored digitally for processing, it is stored as a series of samples. The music on a CD, for instance, might be stored with a .02 millisecond sample interval. That is, the accoustic displacement caused by the sound waves of that music is stored to the CD at 50,000 samples per second.
When a sampled waveform is processed, however, there is an ambiguity as to the original content of that waveform which is caused by aliasing. Aliasing is related to the fact that one sampled frequency can be seen as arising from any of a number of different frequencies.
In this demonstration, you are shown a waveform at the bottom of the screen, its sampled form above that, and a Fourier spectrum of the sampled waveform at the top of the screen. The Fourier spectrum indicates which frequencies are present in a waveform. It can be seen immediately that, although only one frequency was sampled, the spectrum shows two or more peaks.
To the left of the sampled and original waveforms are scroll bars to control the sample interval and signal frequency, respectively. Check out different signal-sample combinations; some are badly aliased, while others are clearer.
Clicking on a peak in the Fourier spectrum will highlight the frequency of that peak and display its waveform in the sample window below. Clicking the same peak again will undo both displays. The peak which is highlighted blue corresponds to the frequency shown in the analog display. Check out some other frequencies -- they all fit just as well as the original.