Electromagnetic wave

All known and measurable energy rays, with the exception of direct particle rays (e.g. electrons or alpha rays), are electromagnetic waves. The diversity of electromagnetic waves is only due to their different wavelengths. Electromagnetic waves include radio waves, radio and television airwaves, mobile phone radiation, microwaves, thermal radiation, light with its various colours, UV radiation, X-rays and gamma radiation.

Characteristics of electromagnetic waves

While radio waves and all radio and television airwaves can have a wavelength of less than one metre up to many metres, the typical wavelength of mobile phone radiation is in the range of around 10 cm, followed by the even shorter wavelengths of microwaves in mm and cm. Between mm and µm, we perceive electromagnetic radiation as thermal energy on the skin. In the range below 1 µm, the radiation then becomes visible as a red colour at around 700 nm and then takes on the colours yellow, green and blue as the wavelength decreases, until waves below 350 nm become invisible again as ultraviolet light. X-rays are shorter than 1 nm and eventually transition to even shorter wave-length gamma rays.

In addition to the wavelength, each wave is characterised by the amplitude, i.e. the height of the wave crests. Here, the square of the wave amplitude is proportional to the intensity of the incident radiation. This means that the amplitude of light waves increases tenfold if the intensity of a light beam is increased by a factor of one hundred.

Electromagnetic waves propagate in a vacuum and by approximation also in air at the vacuum speed of light c = 3•10⁸ m/s. This means that electromagnetic radiation travels 300 000 kilometres in one second, i.e. a distance equivalent to about 7,5 times the circumference of planet Earth at the equator.

Importance of Maxwell's equations

Electromagnetic waves are described mathematically by electrodynamics. With the help of Maxwell's equations, the existence of electromagnetic waves was already predicted and calculated before the existence of electromagnetic waves was even experimentally verified (e.g. by demonstrating the radiation of energy from an antenna in which electrons oscillate back and forth).

By accurately analysing Maxwell's equations, it can be shown that electromagnetic waves consist of oscillating electric and magnetic fields that are perpendicular to one another and generate each other. During propagation, an oscillation of the electric field generates a magnetic field, which in turn generates an electric field.

Propagation of electromagnetic waves

In the following illustration, electric and magnetic waves are shown as rope oscillations that propagate in a certain spatial direction. This is a very simplified depiction. After all, electromagnetic oscillations fill the entire three-dimensional space. The image shown in the illustration is, therefore, to be understood more as the strength of the electric and magnetic fields along an imaginary line.

Electromagnetic waves in quantum theory

Quantum theory has shown that electromagnetic waves only occur as packets with a certain minimum energy. Here, the image of electromagnetic waves was replaced by an image of electromagnetic wave packets that can behave like particles and like waves. The shorter the wavelength, the greater the energy of a wave packet. This is why the quanta of very short-wave X-rays and gamma radiation (electromagnetic radioactive radiation) are very high-energy and correspondingly destructive in their effect on matter.