The Electromagnetic Spectrum

Introduction - Gamma - X - Infrared - Microwaves - Radio - Ultraviolet - Visible


Starting from the smallest wavelengths (highest frequencies) and leading up to the biggest wavelengths, I am going to discuss each type of electromagnetic radiation, giving sources, means of detection and uses.

All these waves travel at the speed of light, that is, 299792459.0ms-1 (± 8). The speed of light is called c for ease of use. The wavelength and frequency of a wave are linked by the equation

c = l f

The shorter the wavelength the greater the penetration.

Gamma Rays

Wavelength: < 10-11m

Sources: Gamma rays are emitted from the nuclei of radioactive atoms during decay. Radioactive decay is spontaneous. This can occur when a neutron splits into an electron and a proton. K-capture also releases gamma radiation. K-capture is the capture of the inner most electron, combined with one of the nucleus' protons, to make a neutron. In both cases, the new products are of a slightly mass than the originals. This mass defect causes a release of gamma radiation (E = Dmc2).

On Earth gamma rays originate largely from cosmic radiation. One high-activity source of gamma rays is cobalt-60 (60Co).

Detection: Bubble chambers, geiger counters (although these are not very sensitive to gamma rays, since the latter are not very ionizing), photographic plates (silver iodide emulsion darkens when hit by gamma radiation).

Bubble chambers use a pressurized tank of H2. High energy particles are sent through the Hydrogen, ionize it, and one can then follow the paths of the bubbles of H2 produced using photographs.

Uses: Gamma rays can be used in many different situations.In radiotherapy they are replacing X-rays from expensive X-ray machines in the treatment of cancer. The rapidly growing cells of the diseased tissue which cause cancer are even more affected by radiation than healthy cells.

Medical instruments, bandages, and such like are sterilized (after packing, to prevent contamination and to make handling easier) by brief exposure to gamma rays. This treatment kills any microbiological organisms which may be on the instruments, thus preventing unintentional spreading of disease. No radioactivity is produced in the material irradiated by the gamma rays and thus this is perfectly safe.

A similar procedure is carried out on foods, extending their "life"-time considerably. For example, meat can be made stay fresh for fifteen days instead of three of four days.

Comments: Gamma rays are the shortest waves we can detect with current instruments. They are very intense, penetrating and dangerous to biological life, and must therefore be handled with care.


Wavelength: 10-11m to 10-9m

Sources: The bombardment of targets of heavy atoms (typically tungsten) by fast moving electrons causes energy levels in the target to change. When the target atoms' excited electrons drop back to their original level, they release fixed quanta of electromagnetic energy. (This is called the photoelectric effect). Basically, X-rays are produced whenever electrons are rapidly brought to rest by matter, however only < 0.5% of the electron's kinetic energy gets converted into these X-rays.

Detection: Photographic plates, fluorescence of certain chemicals (eg, barium platnocyanide), ionization chambers (similar to geiger counters but at a higher pressure).

Uses: The most well known use of X-rays is for medical scans. These are commonly known as "x-rays", this, of course, is incorrect since this is the name of the wave not the method. The method is really called radiography or X-ray photography. This form of detection uses it's fluorescence property.

Another use of X-rays in the medical profession is the use of short wavelength X-rays in a fashion similar to gamma rays for the killing of cancerous cells.

Radiography is also used in industry for the examining of potentially damaged machinery to ascertain the cause of any difficulties, or to verify castings or welded joints prior to distribution.

X-Rays are also used with Bragg diffraction.

Comments: There is no difference between the longest wavelength gamma rays and shortest wavelength X-rays (10-11m). Which name is used usually depends on source and use. X-rays were so called because at first their nature was unknown, for some reason the name stuck once it's nature had been discovered. Short wavelength X-rays are called hard X-rays, long wavelength X-rays are called soft X-rays. X-rays -- just like all eletromagnetic rays -- are not deflected by electric or magnetic fields, and it can thus be deduced that they do not carry a charge.

When X-rays come into contact with atoms they may ionize them (this is cause by the electromagnetic wave's strong electric field). This is why X-rays can be detected in ionization chambers.

Ultraviolet Light

Wavelength: 10-9m to 3.5 × 10-7m

Sources: Ultra hot bodies, mercury vapor lamp, electric arcs (sparks).

The mercury vapor lamp works by photoelectric effect (exciting e- in the mercury and thus releasing photons of the right frequency).

Detection: Photographic plates, fluorescence of certain chemicals, photocells, photoelectric devices.

Uses: UV light produces vitamins (in particular Vitamin D) in the skin and causes sun-tans. Note, though, that UV light is harmful even in modestly large doses. The shorter the wavelength the more dangerous the UV light is. It is used in bacteriology to kill some cells.

Comments: UV light was found shortly after infrared (early 1800s). Much of the UV light emitted by the sun is absorbed by the ozone layer in the Earth's atmosphere. Since our eyes are especially sensitive to UV light, a UV lamp should never be viewed directly. Snow-blindness, which is what skiiers suffer from when skiing on sunny areas, is caused by UV. Manufacturers of washing powders often add fluorescent powders to their products to live up to the claim that their product washes whiter than white, since these powders will absorb UV light and reradiate it as bright visible light.

"Visible" Light

Wavelength: 3.5 × 10-7m to 7.5 × 10-7m

Sources: Very hot bodies (progressively red-hot, yellow-hot and then white-hot), discharge lamps (eg, most bulbs), phosphorence and fluorescence of other types of electromagnetic radiation into visible light.

Detection: Photographic plates, photocells, the human eye.

Uses: Cathode ray tubes, which emit light, are used for televisions, computer monitors and the like. LED displays are used for cheap low resolution visual information. LCDs use the reflection of light for a similar goal. Bulbs are used for lighting which human beings and other animals then use as an aid for (amongst other things) location resolving, navigation, communication, and peripheral/accessory movement (eg, lifting cups of tea).

Apart from all the everyday types of visual communication there are a few other less obvious uses for visible light (which do not depend on it's visible to humans property). One is for high-speed fibre optic links, where red light lasers, green light lasers and (in the future) blue light lasers can carry digital data across long distances at very high speed. With the emergence of the internet these high-bandwidth solutions hold the key to global information sharing since the current infrastructure is not be capable of sustaining the emerging traffic.

Comments: The label "Visible" light demonstrates the ego-centricity of the human race. The short side of infrared and the long side of ultraviolet are separated by an extremely short band (relatively speaking) of radiation which is detectable by the human eye. It is unlikely that another race of intelligent beings, if it had a different natively "visible" section, would highlight the small part between the infrared and ultraviolet as being important.


Wavelength: 7.5 × 10-7m to 10-3m

Sources: All hot bodies.

Detection: Photographic plates, heat sensitive detectors (eg, thermometers), thermopiles.

Uses: Infrared radiation is mainly heat. All moderately hot bodies emit infrared. This is often used to detect human beings (which, being constantly at 310K, are considered hot) by groups like the army (for the killing of humans), and the police (for the prevention of the killing of humans). Firefighters also use infrared detectors to locate the sources of fires (which are naturally hotter than the blazes around them).

Infrared photography also enables pictures to be taken in the dark (no visible light) or in hazy conditions (when visible light is scattered more than infrared).

Infrared radiation is also used in optical fibres, like visible light. This was the original wavelength of the lasers used in this communications method. Infrared is also used for point to point communications.

Infrared is also used for therapeutic purposes.

Comments: The use of Infrared in computer communications is governed by the IrDA standard.


Wavelength: 10-3m to 10-1m

Sources: Special electrical circuits (klystron, magnetron).

Detection: Resonance in similar special electrical circuits (also klystron, magnetron).

Uses: Greatly used in air-borne communications, for example with mobile phones. The shorter the wavelength the greater the bandwidth. Microwaves are also used in point to point "beaming" of energy, it has been suggested that orbiting power satellites could simply "beam" the energy produced to earth based stations via microwave links. However this is still only theory since security precautions would have to be considered to prevent these satellites from being hijacked and turned against civilian areas for the purpose of frying people.

Comments: Note the size of these wavelengths - the longer microwaves' wavelengths are literally measured in centimeters. Contrast this with gamma rays, whose wavelengths are sub-atomic in size. Many stars are microwave emitters.


Wavelength: 10-1m to 104m

Sources: Transmitters (using inductance and capacitance), sparks (eg, from brushes of unsuppressed motors).

Detection: Receivers containing inductance and capacitance which are set into resonance by the wave.

Uses: Medium range air-borne communication.

Comments: There are five main classes of radio waves, they are:

  1. Ultra High Frequency (UHF) - 10-1m to 100m - TV, radar, airport control
  2. Very High Frequency (VHF) - 100m to 101m - FM radio
  3. Short Wave Radio (Ultra High Frequency) - 101m to 102m - Amateur radio, police, ambulances
  4. Medium Wave Radio (Ultra High Frequency) - 102m to 103m - AM radio (national stations)
  5. Long Wave Radio (Ultra High Frequency) - 103m to 104m - AM radio (international stations)