'Electromagnetic spectrum' is the term used to describe the range of electromagnetic waves. The colours of the rainbow, for instance, represent the part of the electromagnetic spectrum that we call visible light. However, visible light is merely a tiny fraction of the whole. For convenience, the spectrum is subdivided into smaller portions according to some of the properties. Keep in mind, though, that the divisions are arbitrary, the borders are blurry and sometimes depend on the context.
An electromagnetic wave is generated in a very simple way. All you have to do is to oscillate a charged piece of something (e.g. electrons or ions) back and forth along a path (e.g. a wire). Charged objects are enclosed in an electric field. Moving a charged object (or changing an electric field) causes a magnetic field. Vice versa, changing or moving a magnetic field causes an electric field. That is, there is a coupling between these two field types. Electric and magnetic fields are able to create each other, and transfer energy to each other in the process. This oscillating energy transfer is an electromagnetic wave. The velocity with which the charged particle moves back and forth is constant for a given medium (eg: a wire or free space). Therefore it is the length of the path which determines the time required for a full cycle of the oscillation. The shorter the path, the more cycles are completed in one second. The number of cycles per second is called the frequency. Thus, electrons oscillating in a short piece of wire operate at a high frequency. Within networks of power landlines, such free oscillation must be prevented to avoid fluctuations in the power rating which could lead to black-outs.
So, charged objects, being accelerated back and forth, emit radiation. This radiation propagates (moves), at the speed of light, in a direction that is perpendicular to the path used by the bouncing piece of charge.
Any type of wave can be represented as an alternating series of 'crests' and 'troughs'. The distance between two crests is called the wavelength. A high frequency wave features more crests per unit length and therefore the distance between them is shorter, ie: high frequency is equivalent to short wavelength and vice versa. Mathematically speaking, frequency is inversely proportional to wavelength. Frequencies are measured in Hertz (Hz), in honour of Heinrich Hertz (1857 - 1894) who proved that James C. Maxwell's equations were not just pure mathematical theory but could be put to practical application.
The electromagnetic spectrum spans from alternating currents to radio waves, microwaves, infrared, visible light up to x-rays and gamma rays. This is equivalent to some twenty orders of magnitude. It took a long time for researchers to acknowledge that radio signals, light and gamma rays are indeed things of the same kind, the only difference being their wavelengths or frequencies. Even today, the lay person is often surprised when being told that ordinary heat is an electromagnetic wave too1. Scanning through the parts of the spectrum listed below, you'll find out that
sound is not an electromagnetic wave (sound is a mechanical vibration)
heat and brain waves are the only electromagnetic waves that humans can create without using tools and machinery
heat and light are the only electromagnetic waves that humans can detect without the help of sophisticated technical devices.
There are some animal species which exhibit astonishing capabilities in this respect:
Bees can see ultraviolet light too, plus they can measure its polarisation (see below)
Some snakes feature 'thermal grooves' below their eyes which serve as detection and homing devices when hunting warm blooded animals.
weakly electric fish can create and detect low frequency electromagnetic waves. The same is supposedly true for sharks.
Fireflies, angler fish and a couple more species can create light. They use it for mating and hunting game.
The following table lists the sections of the electromagnetic spectrum. Note that almost all of it has been put to use for technical appliances but there's a gap of roughly 1THz2 between microwaves and infra-red which has not. This is because the transparency of Earth's atmosphere varies significantly across the whole range of the spectrum. There are areas where radiation is severely attenuated (the THz gap is one of these) and there are atmospheric windows where radition propagates with minor attenuation losses. The most prominent windows are the radio, the infrared and the visual window, where even small signals can be detected from far away. In the following table, 'nm' denotes a nanometre = 10-9m.
|Spectrum of Electromagnetic Radiation|
|Very low freqency (VLF)||> 1013||> 104||< 3·104||< 10-9||broadcasting, submarine communication|
|Radio||1013 - 108||104 - 0.1||3·104 - 3·109||10-9 - 10-5||Radars, telecommunications, magnetic resonance|
|Microwave||108 - 105||0.1 - 0.001||3·109 - 3·1012||10-5 - 0.01||Radars, cooking, telecommunications|
|Infra-red||105 - 700||0.001 - 7·10-6||3·1012 - 4.3·1014||0.01 - 2||Night-vision devices, chemical analysis, heating, thermal imagery|
|Visible||700 - 400||7·10-6 - 4·10-6||4.3·1014 - 7.5·1014||2 - 3||Illumination, lasers, displays, etc...|
|UV (ultra-violet)||400 - 1||4·10-6 - 10-8||7.5·1014 - 3·1017||3 - 103||Artificial sunlight (tanning), chemical analysis, food sterilisation. Blocked by the ozone layer.|
|X-Ray||1 - 0.01||10-8 - 10-10||3·1017 - 3·1019||103 - 105||Diagnostics, Medical X-ray applications, physics/chemistry|
|Gamma Radiation||0.01 - 0.001||10-10 - 10-11||3·1019 - 3·1020||105 - 106||High energy physics, nuclear bombs, cancer treatment|
|Cosmic Radiation||< 0.001||< 10-11||> 3·1020||> 106||(apparently none)|
Every part of the spectrum can be further subdivided into smaller pieces. Some examples are that visible light can be divided into its colours (red, green, blue etc), and you may have seen the terms UV-A and UV-B on sun-screen bottles, which are subdivisions of the ultraviolet part.4 Particularly radio frequencies (RF) are subdivided (e.g. the VHF and UHF ranges one might bump into when programming TV-sets) and are thoroughly controlled by standardisation institutions to keep radio traffic clear of mutual interference. The very low frequency range (VLF) has alternating currents (AC) at its low-energy end, with the typical frequencies of 50 or 60Hz which correspond to really huge wavelengths.
The smallest portion of any electromagnetic wave is called a 'photon'. This might seem confusing at a first glance, since the term 'photon' usually is only found in the context of particles, optical applications or high energy physics. However, all electromagnetic waves are 'made from' photons travelling at the speed of light. In practice, the particle character of a photon is more pronounced at the high-energy part of the spectrum (ie. IR and higher), whereas at lower frequencies the photons are best treated as being waves.
A photon has three fundamental intrinsic properties: energy, polarisation and direction of movement. The energy is equivalent to the frequency. Thus, a photon of visible light carries more energy than a photon of the microwave part but less than a UV photon. The latter are dangerous to humans because their energy is high enough to interfere with the chemistry in a cell by splitting molecules, which can cause skin cancer. X-rays and gamma rays are even more powerful.
The polarisation of a wave denotes a spacial orientation of certain properties of an electromagnetic wave. One very important consequence is that in order to detect a photon, three things must be matched: the detector frequency and the photon frequency, their respective polarisation and the detector direction. In practice this means that a satellite TV dish antenna needs to be
- tuned to the frequency band of the broadcast satellite,
- pointing at the satellite,
- rotated such that the desired (horizontal or vertical) polarisation can be received.
These requirements deserve some more comment:
The pigments in cells responsible for sight are molecules whose length matches the wavelength to which they are most sensitive, and hence they are nothing but antennas for short wavelengths. It's like with dog whistles - no matter how loud the wistle is played, human ears will not hear it because they are not tuned to this frequency. No matter how intense an infrared laser is, humans cannot 'see' it, because our eyes are optimised for the detection of other frequencies. Generalising from there, it could even be argued that the tiny atomic nuclei work like antennas for gamma radiation - and that's why it looks like they're particles.
A liquid crystal display (LCD) emits polarised light. The colour pigments in human eyes are aligned in all polarisations, without any preferences. Therefore you might rotate your LCD screen by 90° and won't notice any difference. Now if you take a polarisation filter and hold it in between, you'll notice that by rotating the filter you can completely block the light. If humans were equipped with bees' eyes, we would have to take care of the screen's orientation.
Last but not least, the intensity of an electromagnetic wave is not an intrinsic property of the photon. Intensity - or brightness - is nothing but the number of photons arriving per second.
NASA tutorial about the EM SpectrumWave-Particle Duality
The electromagnetic spectrum is given by the range of energies (read: frequencies) that photons can have. In order to interact with photons, antennas are needed. The dimensions of an antenna and its orientation are crucial for the detection of photons because the energy and polarisation must match. Thus, antenna sizes grow with the wavelength used. As a consequence, there are typical antennas for each part of the spectrum. For example, radio frequencies are detected with radio or TV antennas, which basically are just wires of different lengths. Walkie-talkies and mobile phones use wires too, but mobile phones operate at higher frequencies so their antennas are so small that they can be hidden within the device. Visible light is detected by exciting the movement of electrons along some long molecules ie: the pigments in the cells of the retina. X-Rays are detected by inducing electron movements in small atoms. Gamma rays are detected by charges moving in the atomic nucleus. In conclusion, the only difference between the different parts of the electromagnetic spectrum is the energy of the photons. That is, they all behave in the same way: They can be absorbed, diffracted and emitted. It is only the scale of length (from atoms and molecules to wires and antenna arrays) in which these processes take place that makes a difference.