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Electromagnetic radiation

Electromagnetic radiation is the name given to a whole range of transverse radiation having differing wavelengths but six common properties, namely:
(a) it is propagated by varying electric and magnetic fields oscillating at right angles to each other;
(b) it travels with a constant velocity of 299 792 458 ms-1 in a vacuum;
(c) it is unaffected by electric and magnetic fields;
(d) it travels in straight lines in a vacuum;
(e) it may be polarised;
(f) it can show interference and diffraction.
The oscillating fields are represented by Figure 1.


For a light beam with an intensity of 100 Wm2 the amplitude of the electric vector can be shown to be 200 Vm-1 and that of the magnetic vector 10-6 T. In optics the electric vector is the more important, partly because of the ability of electric fields to affect static charges.

Regions of the electromagnetic spectrum


For convenience the electromagnetic spectrum is divided into the following regions:

Gamma-rays
[wavelength 10-14 m -10-11 m, frequency 1022 Hz 1019 Hz,
mean energy per quantum 6.6x10-14 J = 4x105 eV = 7.5x10-31 kg]

X-rays
[wavelength 10-12 m -10-8 m, frequency 1020 Hz 1016 Hz,
mean energy per quantum 6.6x10-17 J = 4x102 eV = 7.5x10-34 kg]

Ultraviolet radiation
[wavelength 10-8 m -10-6 m, frequency 1017 Hz 1015 Hz,
mean energy per quantum 6.6x10-20 J = 4x10-1 eV = 7.5x10-37 kg]

Visible light
[wavelength 10-7 m -10-6 m, frequency 1015 Hz 1014 Hz,
mean energy per quantum 6.6x10-19 J = 4x10-2 eV = 7.5x10-38 kg]

Infrared radiation
[wavelength 10-6 m -10-3 m, frequency 1014 Hz 1012 Hz,
mean energy per quantum 6.6x10-21 J = 4x10-4 eV = 7.5x10-40 kg]

Microwaves
[wavelength 10-4 m 10-1 m, frequency 1013 Hz 109 Hz,
mean energy per quantum 6.6x10-23 J = 4x10-6 eV = 7.5x10-42 kg]

Radio waves
[wavelength 10 m 103 m, frequency 108 Hz 106 Hz,
"mean energy" per quantum 6.6x10-26 J = 4x10-9 eV = 7.5x10-45 kg]

Many of these regions overlap, the distinction between one region and another lying in the way in which the radiations are produced. The range of wavelength, frequency and energy per quantum are also shown: the scales for both frequency and wavelength are logarithmic. There follows a summary of the production, properties and detection of the different regions of the electromagnetic spectrum.


Gamma-radiation

This radiation is normally produced by transitions within the excited nucleus of an atom and usually occurs as the result of some previous radioactive emission.

Gamma- radiation can result from fission or fusion reactions or the destruction of a particle-antiparticle pair, such as an electron and a positron. It is used in some medical treatment and also for checking flaws in metal castings, and it may be detected by photographic plates or radiation detectors such as the Geiger tube or scintillation counter.


X- radiation

This occurs due to electron transitions between the upper and lower energy levels of heavy elements, usually excited by electron bombardment or by the rapid deceleration of electrons (known as bremsstrahlung or braking radiation). X-rays are primarily used in medicine and dentistry, and may be detected using photographic film.


Ultraviolet radiation

This is produced by fairly large energy changes in the electrons of an atom. It may occur with either heavy or light elements. The Sun produces a large amount of ultraviolet radiation, most of which is absorbed by the ozone layer in the upper atmosphere.

Ultraviolet radiation will cause fluorescence and ionisation, promote chemical reactions, affect photographic film and produce photoelectric emission. It will also give you a sun tan although since radiation of the required wavelength will not pass through glass you will not go brown unless you are exposed to sunlight directly! Like the preceding radiations it can be dangerous in large doses, particularly to the eyes. Its main uses are in spectroscopy and mineral analysis (some minerals exhibit strong fluorescence under ultraviolet radiation).


Visible light
This is due to electron transitions in atoms. It affects a photographic film, stimulates the retina in the eye and causes photosynthesis in plants.


Infrared radiation

Infrared radiation, discovered around 1800 by William and Caroline Herschel is due to small energy changes of an electron in an atom or to molecular vibrations. It may be detected by a thermopile or special photographic film. Since it is less scattered by fine particles than visible light (be- cause of its longer wavelength) infrared radiation is useful for haze photography. It is also used by Earth resource satellites to detect healthy crops; most of us are familiar with its use for heating, both in the home and in hospitals. It may be refracted by rock salt.

Microwaves

These are produced by valves such as a magnetron or with a maser. They are used in radar, telemetry and electron spin resonance studies and in microwave ovens. In a microwave oven the food is heated because it contains water that is a strong absorber of microwaves. The microwaves excite the water molecules, the velocity of the molecules rises and therefore the temperature of the food rises. This explains why the food is heated but the temperature of the containers does not rise very much. Microwave ovens are useful because they reduce cooking time considerably since they cook the food from within.

Microwaves may be detected with crystal detectors or solid-state diodes. The radiation from interstellar hydrogen has a wavelength of 21 cm (0.21 m) and so lies at the edge of the microwave region: the detection and analysis of this radiation has added greatly to our knowledge of the structure of the universe.

Radio waves

These waves have the longest wavelengths of any region of the electromagnetic spectrum and therefore the smallest frequency and hence the lowest energy per quantum. They are produced by electrical oscillations and may be detected by resonant circuits in radio receivers. Their use is of course in radio and television communications.

Information may be transmitted on an electromagnetic wave. Initially we have a wave, say a radio wave, known as the carrier wave. The carrier wave is then modulated, either in amplitude (Figure 2(a)) or frequency (Figure 2(b)) by the signal .



In amplitude modulation amplitude is varied with time while with frequency modulation it is the frequency that is changed.

The velocity of electromagnetic waves in free space (i.e. a vacuum with no boundaries) is given by the following equation:

Velocity of electromagnetic radiation in free space = 1/[εoμo]1/2


where εo is the permittivity of free space and μo is the permeability of free space.

This equation is used to find εo.
 

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© Keith Gibbs