<P> A waveform is the shape and form of a signal such as a wave moving in a physical medium or an abstract representation . </P> <P> In many cases, the medium in which the wave propagates does not permit a direct observation of the true form . In these cases, the term "waveform" refers to the shape of a graph of the varying quantity against time . An instrument called an oscilloscope can be used to pictorially represent a wave as a repeating image on a screen . </P>

The most common wave forms of ac is