The Electromagnetic Spectrum

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Electromagnetic energy (EME) is a natural form of energy and occurs in many different forms as shown in the electromagnetic spectrum above. The largest natural source is the Sun and this generates energy in many parts of the EM spectrum, including radio signals. EME is composed of oscillating (changing) electric (E) and magnetic (M) fields – EM fields.

These fields are characterised by frequency (in units of Hertz – Hz) and wavelength (in units of metre). The frequency is the number of crests or troughs that pass by a point in space in one second. The wavelength is the distance between two peaks or two troughs of an electromagnetic wave.

The frequency and wavelength are interrelated by the velocity of light in free space, which has a value of 3x10⁸ ms-¹ (metres per second). This relationship is given by the following equation: velocity = frequency x wavelength. For example the domestic microwave oven uses a frequency of 2.45 GHz (2,450,000,000 Hz), which corresponds to a wavelength of about 0.12 m (12 cm). More information on metric or SI prefixes can be found here.

Fields at different frequencies within the EM spectrum behave differently in their interactions with matter. Static magnetic fields are associated with magnetic resonance imaging (MRI) machines used for certain scans. Extremely low frequency (ELF) fields are produced by electrical wiring and equipment. These primarily induce currents in the body that may stimulate nerves or muscles. For about 100 years a range of frequencies have been used for communication, radio frequency (RF) signals. RF signals at higher frequencies are sometimes termed microwaves – a reference to their short wavelength. Radio signals primarily cause heating of the body, internal to the body at low frequencies and closer to the skin at higher frequencies. Infrared (IR) energy may be sensed through thermal receptors in the skin and is associated with electric bar heaters. A very narrow band of frequencies can be seen as light and may induce photochemical effects, for example, photosynthesis in plants. Ultra-violet (UV) energy triggers the tanning response in skin but about halfway through the UV part of the spectrum there is sufficient photon (or intrinsic) energy to cause ionization, that is, to break molecular bonds. If these bonds form part of genetic material, ionizing energy may be able to cause direct genetic damage, thus initiating cancer. At higher frequencies still there are more penetrating forms of energy, such as used for medical x-rays or produced in nuclear reactions.

Another term to be defined is radiation - the transfer of energy through space. It does not indicate whether the energy is ionizing or non-ionizing. Scientists thus sometimes refer to electromagnetic radiation (EMR) or radiofrequency radiation (RFR).

It is important to note that RF signals are a form of non-ionizing energy.