The electromagnetic spectrumFrom Physclips: Physics with animations and film.

Electromagnetic waves fill a spectrum with wavelengths from thousands of kilometres long down to wavelengths more than 1020 times smaller. They may be detected using a range of quite different instruments. As the graphic shows, visible light comprises only a tiny fraction of this spectrum: less than an octave. Photon energies also vary over this huge range: in the radio band we collect huge numbers of photons, each having only a tiny energy. The phase of the photons in a radio transmission is not random: it is such that their fields add together, and we can therefore observe their combined electric and magnetic fields as they oscillate in time and space. For gamma rays, we may observe the effects of many charged particles, all created by a single photon.

This page discusses the uses and properties of the different bands, and several of the important concepts associated with electromagnetic waves.

In one classification system, the waves used for radio communication (and other purposes) are neatly divided up in decades, ie divided into bands whose wavelengths and frequencies vary over a factor of 10. In wavelength, the bands begin and end on metres times a power of ten. Because the speed of light is close to 3 10 8m/s, when these bands are expressed in frequencies, their limits are 3 times a power of 10 Hz. eg for 3 GHz, λ = c/f = 10 cm. The names of the bands are:
• Sources in the Super Low and Extra Low Frequency bands (SLF and ELF) are mainly accidental or natural. For instance, electricity authorities have very long antennae, called power lines, that radiate at 50 or 60 Hz. This signal is picked up as 'hum' and is cursed by electrical engineers everywhere. A large natural source is the interaction of the solar wind with the ionosphere that produces low frequency currents (telluric currents) in the earth and oceans, and these are studied by geophysicists to deduce, inter alia, the presence of ore bodies whose electrical conductance differs from that of the surrounding crust. Like ULF, these bands may be used for communication with submarines, with low information rates.
• 300 Hz - 3 kHz. Ultra Low Frequency (ULF). Electromagnetic waves in this range are not strongly absorbed by water or the earth. They may therefore be used to communicate with submarines and with mines. One disadvantage is that, with such low frequency, one can only modulate their amplitude or frequency very slowly (eg with morse code) so they cannot carry much information. This is not a disadvantage if only the phase is required, as is the case for navigation systems. The wavelengths are so long that antennae may be huge.
• 3 - 30 kHz. Very Low Frequency (VLF). Again, the information carrying capacity is limited. These are used in navigation systems.
• 30 - 300 kHz. Low Frequency (LF). This band has the advantage that it can propagate around the Earth, by refraction and reflection at the ionisphere or the surface of the Earth itself. Indeed, these two conductors form a waveguide for waves in this range, which can therefore be used to communicate across the oceans and around the world.
• 300 kHz - 3 MHz. Medium Frequency (MF). (This includes the AM radio band: see below). These waves are not so well reflected/refracted by the ionosphere, but at night there is enough reflection that one can pick up radio stations hundreds or thousands of km away. This is not possible with the much shorter waves used for FM radio or television: for these you need an unobstructed path to the transmitter that is not much different from a straight line.
• 3 - 30 MHz. High Frequency (HF). This is also known as the Short Wave band. It includes the CB band (see below) and the channels used for radio control. As the frequency of the carrier wave increases, it becomes possible to encode more information and to crowd channels (proportionately) closer together.
• 30 - 300 MHz. Very High Frequency (VHF). (includes FM radio and television). Antennae are often made to be about one quarter or one half wavelength long.
• 300 MHz - 3 GHz. Ultra High Frequency (VHF). (GHz = 109 Hz). This includes some television and mobile phones: see below. Many channels are available.
• 3 - 30 GHz. Super High Frequency (SHF). (roughly corresponds to microwave band) Used for communication with satellites.
• 30 - 300 GHz. Extra High Frequency (EHF). Not much used for radio communication (yet), because of the technological difficulty of encoding and decoding amplitude and frequency modulation at such high frequencies.
Here ends the radio band. Hereafter, wavelengths are used almost exclusively, partly for traditional reasons, and partly because frequencies in the THz range (THz = 1012 Hz) are difficult to measure directly. (They can be measured by heterodyning: observing the difference frequencies they make with reference signals.)

Infrared, visible, ultraviolet, X and gamma rays

• Infrared: wavelengths longer than visible and up to about 1 mm (often measured in microns or micrometres, symbol μm). Infrared radiation can be felt as radiant heat: eg when you stand in front of a fire. Some snakes have IR sensors. The military uses IR binoculars for the same reason as snakes do: to find mammals, who are usually warmer than our surroundings.

• Visible: Wavelengths are about 400 nm (violet light) to 700 nm (red light). A nanometre, symbol nm, is 10-9 m. The sun radiates most strongly in this range, and our atmosphere does not absorb it (Los Angeles excepted). This is not a coincidence: we have evolved on this planet in this atmosphere, so of course we have evolved sensors that use the available radiation. (Pace Drs Pangloss, Liebniz and certain other naifs. Any readers interested in teleology should follow this link.) Visible light can cause chemical reactions (eg vision and photosynthesis) but usually does not. The diodes used in solar cells work at a potential difference of about 0.6 volts, so every visible photon has enough energy to shift one electron across the interface. See the introductory page on the photovoltaic effect.

• Ultraviolet: wavelengths shorter than visible, down to about 10 nanometres. UV is more useful in chemistry, because each photon has energy comparable with that of a chemical reaction. It is dangerous for the same reason: a UV photon has enough energy to damage DNA molecules in your cells (so remember to wear sunscreen and a hat). If objects are hotter than the sun (eg some massive young stars), they radiate in the UV. Bees can see in the near UV and so flowers have UV colours to attract them. (Bees can also see the polarisation of light, which they use to navigate, but that's another story.) When the energy of photon is high enough, it is often expressed in electron volts: an ultraviolet photon with 10 eV or more has enough energy to ionise an atom if its outer electron is held at an electric potential of 10 Volts. This is typical of the binding energies of atoms, which is why UV is chemically potent.

• X rays: wavelengths from several nm to 10 pm (a picometre is 10-12 m). Xrays with wavelengths comparable to atom dimensions are used to determine the structure of crystals, in a technique developed by the Australian physicists, William and Lawrence Bragg, for which they received the Nobel prize in 1915. X rays are divided informally into 'soft' X rays with long wavelengths and 'hard' X rays with shorter wavelengths and higher energies. Their energies are enough to ionise atoms and to destroy chemical bonds. They are produced naturally by some radioactive sources, or by very hot objects like neutron stars. They are also produced by smashing high energy electrons into metal targets: X rays thus produced are used to treat cancers including breast cancer. Soft X rays are stopped by (enough) air. Hard X rays can penetrate deeply into tissue.
• Gamma rays: wavelengths less than about 10 pm. They have very high energy, and often come from deep space, sometimes in bursts from cataclysmic cosmic events, such as the collapse or collision of stars. A 10 GeV cosmic ray has the same energy as an electron would have it were accelerated through 10 billion volts. This is enough energy to cause a chain reaction of ionisation events in the Earth's atmosphere, leading to a shower of charged particles.

The electromagnetic spectrum

Common names for radio bands. For practical purposes, other divisions of the radio part of the spectrum are used, including those bands allotted for specific types of communication. So for instance people talk of the AM radio band, of the CB band etc. Here are some examples:

• AM radio: 535 - 1,700 kHz (0.535 - 1.7 MHz) Have a look at the dial on your radio and check the frequency of your favourite AM station. Then divide this into the speed of light to get the wavelength. Fortunately, you do not need an antenna that has a comparable length, although the strength of the signal will increase as you increase the antenna length.

• Short wave - several different bands in the range 5.9 - 26.1 MHz

• Citizens band (CB) radio - Several bands around 27 MHz.

• FM radio: 88 - 108 MHz. If the announcer says 102.5 FM, she is telling you the frequency of her station. The wavelength are about 3 metres, so simple antennae should be about 1/4 or 1/2 this length. To get an idea of how crowded the EM spectrum is, have a look at this scan (click on the yellow graphic) provided by Balint Seeber, a rather special physics student at UNSW.

• Television - several different bands between 54 and 220 MHz. (Television carries more information than radio does--pictures plus sound-- and so needs broader bands for each channel)
• Mobile phones: 824 - 849 MHz
• Global Positioning System: 1.2 -1.6 GHz
• The microwave band is used less formally for wavelengths of cm down to mm, or frequencies up to 10s or 100s of GHz. The microwave band is used for radar and long distance trunk telephone communications. Domestically, it is also used in microwave ovens.
* A FAQ about microwave radiation is whether that produced by a portable telephone can do damage to the brain to which it may be rather close. The evidence on this is still not clear. A discussion is at given in "Microwave Radiation and Leakage of Albumin from Blood to Brain", James C Lin, IEEE Microwave Magazine, September 2004.

Measurement

Measurement techniques, as well as the uses, vary considerably over the range. At long wavelengths and low frequencies, we can observe precisely how the electric and magnetic field vary with time. At the lowest frequencies, we can measure the time per cycle: at high frequencies, the number of cycles per unit time. In high GHz or Thz regime, we can no longer measure frequency directly, although we can calculate it from the wavelength and the speed, or measure it using indirection means such as heterodyning. Wavelenths are usually measured using spectrometers, which use the phenomenon of interference. For X rays, the diffraction gratings in the spectrometers are crystals. For gamma rays, whose wavelengths are rather smaller than atomic dimesions, all we can measure is the energy.

Wave vs particle vocabularies for EM radiation

The different limitations involved in measurements have implications for our choice to use phrases from the wave vocabulary or the particle vocabulary to describe radiation. For instance, if we are talking about a transmitted radio wave in the medium wave band, then huge numbers of photons would combine to make an electric and a magnetic field whose amplitude we could measure fairly accurately. The intensity of this wave would be proportional to the square of the amplitude of the electric field (or the square of the amplitude of the magnetic field). We would not talk about photons, because it is virtually impossible to measure them individually: they each have less energy than the kinetic energy of atoms and electrons due to their thermal motion. We could not distinguish photon capture from the random thermal motion of electrons in our detector. Even if we cool a detector down to microKelvin temperature (see graphic) to try to measure photons one at a time, their energy is so small that it is a difficult task. (Measuring the energy in radio waves is like measuring water by volume: the molecules of water are there, but there are very many molecules in every drop so we think of water as a continuum.)

This radio wave is also different from from ordinary light because it it is polarised, and because it has a very long coherence length: that is we can relate the phase predictably over regions of the wave separated by many km. Further, it is possible to measure and to display the electromagnetic fields (or rather the voltages they produce in an antenna) as a function of time. These measurement possibilities dispose us to use the vocabulary of waves to describe the phenomena.

Light

On the other hand, for light or for waves with shorter waves, we cannot measure or display E(t): the fields oscillate too fast. Instead, with light, we 'catch photons': a single photon interacts with a photoreceptor molecule in your eye, a crystal in a film, an electron in a photocell/photomultiplier tube etc. Because this is localised in space and time, we are using the particle vocabulary. In this vocabulary, the intensity of the wave is the energy per photon times the number of photons per unit area.

Notice that the choice to use wave or particle vocabulary has been made according to what we can measure (or sometimes what is convenient to discuss). (It is the opinion of this author that little insight is gained from talking about wave-particle 'duality' or whether EM radiation 'is' a wave or a collection of particles. Such talk may, however, help sell popular science books.)

Photons and chemistry

Ultraviolet light causes sunburn but visible does not. Why so? Many chemical reactions may be activated by electromagnetic radiation. In the simplest case, one photon interacts with one molecule to initiate the reaction. Each photon has an energy hf, where h is Planck's constant, 6.63 X 10−34 J.s = 4.14 X 10−15 eV.s.

A hydrogen atom has an ionisation energy of about 13 eV so, looking at the spectrum table above, a photon with a wavelength not much shorter than 100 nm (well out in the ultraviolet) has enough energy to ionise a hydrogen atom. Familiar chemical reactions have reaction energies of tens of kJ per mol. Let's take 50 kJ.mol−1 as a reaction energy, divide it by Avagadro's number (6 X 1023 to obtain a value per molecule, and use 1.6 X 10−19 eV per joule to obtain about 0.5 eV per molecule as a reaction energy. So, if it were just a question of getting from initial to final state, a photon in the infrared could supply the energy. Usually, however, there is an actived state with a rather higher energy, so more energy is needed.

Visible light can cause some reactions – such as the photochemistry in our eyes, or on photographic film. Photosynthesis is another (rather complicated) example. Ultraviolet light has more energy available, so UV can cause sunburn, while visible light does not. Hard UV can break carbon-carbon bonds and have serious biochemical effects for people.

Entropy

The (change in) entropy is defined as the heat added reversibly to a system, divided by its temperature. Usually, heat and radiation go from low entropy (high T) to high entropy (low T). For example, in a kitchen grill, infrared radiation at several hundred K (and some weak red light) is transmitted to food at lower temperature (a few hundred K).

This may seem to raise a paradox: microwaves have energies of meV, yet in a microwave oven they are used to heat food whose molecules already have thermal energies of ~0.1 eV. The point here is that the intensity of the radiation produced by the magnetron or klystron in the microwave oven is much greater than that of its thermal radiation. Putting your food in interstellar space, where the microwave radiation is weak, would not cook it: it would simply cool to about 3 K. Further, the radiation produced by a magnetron (or by a radio transmitter) is not random, whereas thermal radiation is random. Transmitters usually produce photons that all have nearly the same phase. For example, a sufficiently intense but low frequency electric field could produce an electric field of magnitude 100 MV/m, which is enough to ionise atoms, even though one photon might not have nearly enough energy for ionisation. The field is strong because all of the photons are in phase and we have a low entropy source. This brings us to the relation between entropy and information.

Information

Just like the waves produced by a microwave oven, the radio waves used for communication consist of huge numbers of photons, all very nearly in phase. This gives them a much lower entropy than that of a similar number of photons with random phase. We can then vary the photon phase (usually in the very slight ways associated with amplitude and frequency modulation) so as to carry useful information.

Sources whose photons have random phase carry information in other ways. Astronomers use waves from radio to gamma rays to make images of the sky. To do this, a minimum of several photons (and usually many more) must be averaged for each pixel in the image. Under optimal, dark adapted conditions, a single human photoreceptor must capture several photons in a tenth of a second to be excited and to give us the sensation of a weak flash of light. Our eyes are at best about 10% efficient, so this requires us to receive at the cornea several dozen photons focussed onto one point in the retina. Charged Coupled Detectors are used in cameras and they are considerably more efficient than our eyes, especially CCDs operating at very low temperatures.

History

This page is a distillation of the work of many people who have worked to understand electromagnetism, light and heat. Thinking about light is an essay by physics teacher Russell Downie on the history of our understanding of light.

 Joe Wolfe © 2002. Modified May 03 J.Wolfe@unsw.edu.au, phone 61- 2-9385 4954 (UT + 10, +11 Oct-Mar). School of Physics, University of New South Wales, Sydney, Australia. There are pages on related material at

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