|Electromagnetic waves fill a spectrum with wavelengths from thousands of kilometres long down to wavelengths more than 1020times 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.
Standard names for radio bands
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:
Here ends the radio band. Hereafter, wavelengths are used almost exclusively, partly for traditional reasons, and partly because frequencies in the THz range (THz = 1012Hz) are difficult to measure directly. (They can be measured by heterodyning: observing the difference frequencies they make with reference signals.)
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 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 wavein 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.
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.)
Temperature and colour
|When photons with a given energy equilibrate with matter, the thermal energy of the atoms (or electrons, etc) is comparable with that of the photons. A body in equilibrium with its radiation is called a black body, and the wavelength at which a black body with (absolute) temperature T has its greatest radiant power is given by Wien’s displacement law:
(See Black body radiation for more details. There is also a page on thermal radiation and why clothes work.) Thus the sun, whose surface approximates a black body with temperature 5,700 K, has maximum radiation at about 500 nm, in the middle of the visible range. It also emits wavelengths on either side, and this combination is what we call white light. A hotter star (or a welding spark) emits proportionately more shorter wavelengths and so appears blue. A cooler star (or a normal fire) emits mainly longer wavelengths, and so appears red.So, if the sun has peak radiation in the green, why doesn’t it look green? The answer has to do with bandwidth (which is defined as the difference between the frequencies that have half the power of the maximum, one on either side). The whole visual bandwidth is less than an octave: from violet to visible red the wavelength change is less than 100%. The bandwidth of each of our photo receptor types (formally named L for long, M for medium and S for shorb, but more commonly known as R, G and B) is about 20%. The wavelengths of maximum sensitivity for the three types of photo receptor are 440, 545 an 565 nm, and the plot shows black body radiancy for these temperatures.
As the plot of black body radiation shows, the bandwidth (frequency range between points of half maximum power) of a hot body is rather more than 100%. Looking at this curve, you will see that a star (or other simple hot body) with maximum radiation in the green emits very strongly in red, green and blue. In the case or the sun, or most 5700 K bodies that are close to us, the intensity is great enough that it will saturate all three colour receptor types, so that we see white. So how can we see red and blue stars? The edges of the peaks in the curve are steep. When we see a blue star, its maximum is in the UV, and red and orange stars have theirs in the IR. (again, have a look at the curve). One star with a maximum in the green is the sun. Now you’re not supposed to look at the sun when it is overhead, but I did (very briefly) and it is white, due to saturation of all photoreceptors. (The other colours it has near sunrise and sunset are due to atmospheric scattering or, in the case of the green flash, due to scattering plus dispersion.)
The background radiation of the universe has a temperature of a 3 K (or -270°C), and so its spectrum is mainly in the microwave range. Because we can’t see microwaves, it therefore looks ‘black’ or invisible to us: it is the radiation coming from the night sky where there are no stars. This radiation has been travelling through space ever since the universe was a few hundred thousand years old, when it first became electromagnetically transparent. The universe was much hotter then, but because it has expanded a lot, its radiation has expanded too (wavelengths have become longer) and become much cooler.
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.
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.
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.
Source : Spektrum Elegtromagnetik