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.
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.
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.
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).
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.
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 .