Sources of electromagnetic waves

Charges at rest produce only electrostatic fields. Charges moving uniformly (steady currents) produce magnetic fields that do not change with time. Neither is a source of electromagnetic waves.

Accelerated charges radiate electromagnetic waves. This follows from Maxwell's theory; the proof is beyond the syllabus.

An oscillating charge is an accelerating charge. It sets up an oscillating electric field, which sets up an oscillating magnetic field, which in turn sets up an oscillating electric field, and so on: the two fields keep regenerating each other as the wave moves out.

The frequency of the wave equals the frequency with which the charge oscillates. The energy the wave carries comes from the source, the accelerated charge. An oscillating electric dipole is a basic source.

Testing Maxwell with an ac circuit at the frequency of light is impossible: yellow light has a frequency of about 6 × 10¹⁴ Hz, while even modern electronic circuits barely reach about 10¹¹ Hz. So the first demonstration used low-frequency radio waves.

Hertz produced and detected electromagnetic waves in the laboratory in 1887, with wavelengths of the order of a few metres, confirming Maxwell's prediction.

Seven years after Hertz, Jagdish Chandra Bose, working in Calcutta (now Kolkata), produced and observed much shorter waves, 25 mm to 5 mm, also within the laboratory.

Around the same time Guglielmo Marconi in Italy sent electromagnetic waves over many kilometres, the start of communication by electromagnetic waves.

The wavelength radiated often matches the size of the radiating system: an aerial radiates best at a wavelength about its own length, and nuclei give gamma rays of 10⁻¹⁴ to 10⁻¹⁵ m. Visible light from atoms is the exception, much longer than an atom.