Wave Shapes
The top image represents the 'shape' of the radio waves generated by scratching
the battery on the previous page or those generated inside your computer. Because
the time taken for the wave to switch from minimum to maximum intensity (and
vice versa) is very short, it can be shown mathematically that this type of wave
actually consists of many, many waves of different frequencies - which is why they
can be heard wherever you tune your radio.
Contrast that with the lower picture showing a pure sine wave of a single, precise frequency. This is the sort of wave we need in order to produce useful (intentional) radio waves because a virtually unlimited number at different frequencies can be produced each of which can be received separately by a suitable radio receiver.