Since humans normally can't directly observe invisible radiations such as electromagnetism, an agreement as to their structure is based on instrumentation applied to electrical circuits. From that, we see certain patterns form on instrument readouts, and we can refer to these patterns as the form of the wave being observed. In many cases, what we are seeing on the readout may only be a cross-section of part of the wave.

In cases where instrumentation is not available to provide a readout of a particular phenomenon, any illustration of the form of the waves involved must be regarded as a good guess.