ELECTROMAGNETIC SPECTRUM

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Early in nineteenth century a scientist called William Herschel discovered that if a sensitive thermometer was held in the various colours of the spectrum, the reading on the thermometer went up. This shows that all colours of light produce some heat. When Herschel held the thermometer just beyond the red end of spectrum the reading on the thermometer suddenly shot up. His discovery suggested that there were invisible rays with longer wavelengths than that of visible light. These produced more heat than the rays of visible light. These rays are called infra-red rays. All bodies above absolute zero give off these rays.

To detect infra-red rays (radiant-heat) we use a thermocouple. These rays can be refracted just as light rays. Other invisible rays exist whose wavelength is just a little shorter than that of violet light. They are called Ultraviolet rays and have several uses. They kill germs in operating rooms, they enable the skin to manufacture vitamin D, and they cause certain chemicals to shine brightly even in the dark.

Other waves which have wavelengths longer than those of infra-red are radio waves, X-rays and gamma rays. These have much shorter wavelengths than that of ultraviolet. All these radiations, including light, form what is called “electromagnetic spectrum”. Their sources and properties are discussed in details below.

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The “electromagnetic spectrum” of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.

The electromagnetic spectrum extends from low frequencies used for modern radio to gamma radiation at the short-wavelength end, covering wavelengths from thousands of kilometres down to a fraction of the size of an atom. The long wavelength limit is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length, although in principle the spectrum is infinite and continuous.

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The electromagnetic spectrum and sources of each type of radiation

INFRARED RADIATION

The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz (1 mm) to 400 THz (750 nm). It can be divided into three parts:

1.       Far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm). The lower part of this range may also be called microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by phonons in solids. The water in the Earth’s atmosphere absorbs so strongly in this range that it renders the atmosphere effectively opaque. However, there are certain wavelength ranges (“windows”) within the opaque range which allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as “sub-millimetre” in astronomy, reserving far infrared for wavelengths below 200 μm.

2.       Mid-infrared, from 30 to 120 THz (10 to 2.5 μm). Hot objects (black-body radiators) can radiate strongly in this range. It is absorbed by molecular vibrations, where the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the fingerprint region since the mid-infrared absorption spectrum of a compound is very specific for that compound.

3.       Near-infrared, from 120 to 400 THz (2,500 to 750 nm). Physical processes that are relevant for this range are similar to those for visible light.

Our bodies detect infrared radiation (IR) by its heating effect on the skin. It is sometimes called ‘radiant heat’ or ‘heat radiation’.

Anything which is hot but not glowing, i.e. below 500°C, emits IR alone. At about 500°C a body becomes red-hot and emits red light as well as IR – the heating element of an electric fire, a toaster or a grill are example. At about 1500°C things such as lamp filaments are white-hot and radiate IR and white light, i.e. All the colours of the visible spectrum.

Infrared is also detected by special temperature sensitive photographic films which allow pictures to be taken in the dark. Infrared sensors are used on satellites and aircraft for weather forecasting, monitoring of land use, assessing heat loss from buildings and locating victims of earthquakes.

Infrared lamps are used to dry the paint on cars during manufacture and in the treatment of muscular complaint. Remote control keypads for televisions contain a small infrared transmitter for changing programs.

ULTRAVIOLET RADIATION

This is radiation whose wavelength is shorter than the violet end of the visible spectrum, and longer than that of an X-ray.

Ultraviolet (UV) rays have shorter wavelengths than light. They cause sun-tan and produce vitamins in the skin but can penetrate deeper, causing skin cancer. Dark skin is able to absorb more UV so reducing the amount reaching deeper tissues. Exposure to the harmful UV rays present in sunlight can be reduced by wearing protective clothing such as a hat or using sunscreen lotion.

Ultraviolet causes fluorescent paints and clothes washed in some detergents to fluoresce. They glow by re-radiating as light the energy they absorb as UV. This may be used to verify ‘invisible’signatures on bank documents.

A UV lamp used for scientific or medical purposes contains mercury vapour and this emits UV when an electric current passes through it. Fluorescent tube also contains mercury vapour and their inner surfaces are coated with special powder called phosphors which radiate light.

Being very energetic, UV can break chemical bonds, making molecules unusually reactive or ionizing them, in general changing their mutual behaviour. Sunburn, for example, is caused by the disruptive effects of UV radiation on skin cells, which is the main cause of skin cancer, if the radiation irreparably damages the complex DNA molecules in the cells (UV radiation is a proven mutagen). The Sun emits a large amount of UV radiation, which could quickly turn Earth into a barren desert. However, most of it is absorbed by the atmosphere’s ozone layer before reaching the surface.

RADIO WAVES

Radio waves have the longest wavelength in the electromagnetic spectrum. They are radiated from aerials and used to carry sound, pictures and other information over long distances.

1.       Long, medium and short waves (wavelength of 2 km to 10m)

These diffract round obstacles so can be received when hills etc. Are in their way. They are also reflected by layers of electrically charged particles in the upper atmosphere (the ionosphere), which makes long-distance radio reception possible.

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Figure: 9.74:  Diffraction and reflection of radio waves

2.       VHF(Very High Frequency) and UHF (Ultrahigh Frequency) waves (wavelength of 10m to 10cm)

These shorter wavelength radio waves need a clear, straight-line path to the receiver. They are not reflected by the ionosphere. They are used for local radio and for television.

3.       Microwaves (wavelengths of a few centimetres)

These are used for international telecommunications and television relay via geostationary satellites and for mobile phone networks via microwave aerial towers and low-orbit satellites. The microwave signals are transmitted through the ionosphere by dish aerials, amplified by the satellite and sent back to dish aerials in another part of the world.

Microwaves are also used for radar detection of ships and aircraft, and in police speed traps.

Microwaves can be used for cooking since they cause water molecules in the moisture of the food to vibrate vigorously at the frequency of the microwaves. As a result, heating occurs inside the food which cooks itself.

Living cells can be damaged or killed by the heat produced when microwaves are absorbed by water in the cells. There is some debate at present as to whether their use in mobile phones is harmful; hands-free mode, where separate earphones are used, may be safer.

The super high frequency (SHF) and extremely high frequency (EHF) of microwaves come next up the frequency scale. Microwaves are waves which are typically short enough to employ tubular metal waveguides of reasonable diameter. Microwave energy is produced with klystron and magnetron tubes, and with solid state diodes such as Gunn and IMPATT devices. Microwaves are absorbed by molecules that have a dipole moment in liquids. In a microwave oven, this effect is used to heat food. Low-intensity microwave radiation is used in Wi-Fi, although this is at intensity levels unable to cause thermal heating.

Volumetric heating, as used by microwaves, transfers energy through the material electromagnetically, not as a thermal heat flux. The benefit of this is a more uniform heating and reduced heating time; microwaves can heat material in less than 1% of the time of conventional heating methods.

When active, the average microwave oven is powerful enough to cause interference at close range with poorly shielded electromagnetic fields such as those found in mobile medical devices and cheap consumer electronics.

X-RAYS

These are produced when high-speed electrons are stopped by a metal target in an x-rays have smaller wavelengths than UV.

They are absorbed to some extent by living cells but can some solid objects and affect a photographic film. With materials like bones, teeth and metal which they do not pass through easily, shadow pictures can be taken, like that of someone shaving. In industry, x-ray photography is used to inspect welded joints.

X-ray machines need to be shielded with lead since normal body cells can be killed by high doses and made cancerous by lower doses.

GAMMA RAYS

After hard X-rays come gamma rays, which were discovered by Paul Villard in 1900. Gamma rays are more penetrating and dangerous than x-rays. These are the most energetic photons, having no defined lower limit to their wavelength. They are useful to astronomers in the study of high energy objects or regions, and find a use with physicists thanks to their penetrative ability and their production from radioisotopes. Gamma rays are also used for the irradiation of food to kill harmful bacteria and seed for sterilization, on surgical instruments and in medicine they are used in radiation cancer therapy to kill cancer cells and some kinds of diagnostic imaging such as PET scans. The wavelength of gamma rays can be measured with high accuracy by means of Compton scattering.

Note that there are no precisely defined boundaries between the bands of the electromagnetic spectrum. Radiations of some types have a mixture of the properties of those in two regions of the spectrum. For example, red light resembles infrared radiation in that it can resonate some chemical bonds.

 VISIBLE RADIATION (LIGHT)

Above infrared in frequency comes visible light. This is the range in which the sun and stars similar to it emit most of their radiation. It is probably not a coincidence that the human eye is sensitive to the wavelengths that the sun emits most strongly. Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another. The light we see with our eyes is really a very small portion of the electromagnetic spectrum. A rainbow shows the optical (visible) part of the electromagnetic spectrum; infrared (if you could see it) would be located just beyond the red side of the rainbow with ultraviolet appearing just beyond the violet end.

Electromagnetic radiation with a wavelength between 380 nm and 760 nm (790–400 terahertz) is detected by the human eye and perceived as visible light. Other wavelengths, especially near infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes referred to as light, especially when the visibility to humans is not relevant.

If radiation having a frequency in the visible region of the EM spectrum reflects off an object, say, a bowl of fruit, and then strikes our eyes, this results in our visual perception of the scene. Our brain’s visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood psychophysical phenomenon, most people perceive a bowl of fruit.

At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum, and our technology can also manipulate a broad range of wavelengths. Optical fibre transmits light which, although not suitable for direct viewing, can carry data that can be translated into sound or an image. The coding used in such data is similar to that used with radio waves.

In summary, you can use the following technique to master all the components of the electromagnetic spectrum, according to the order of frequency and wavelength.

Roast

Maize

Is

Very

Unusual

X-mas

Gift

Radio waves

Microwaves

Infrared

Visible light

Ultraviolet

X-ray

Gamma rays

Low frequency /Long wavelength

High frequency/ short wavelength

The mnemotechnique to remember the spectrum arrangement.

 PROPERTIES OF ELECTROMAGNETIC RADIATION

Light is one member of a family of electromagnetic radiation, which forms a continuous spectrum beyond both ends of the visible (light) spectrum. While each type of radiation has a different source, all result from electrons in atoms undergoing an energy change and all have certain properties in common:

1.     All types of electromagnetic radiation travel through space at 300000km/s ( ), i.e. with the speed of light;

2.       They exhibit interference, diffraction and polarization, which suggests they have a transverse wave nature;

3.     They obey the wave equation  where v is the speed of light; f is the frequency of the wave and  is the wavelength. Since v is constant for a particular medium, it follows that large f means small .

4.       They carry energy from one place to another and can be absorbed by matter to cause heating and other effects. The higher the frequency and the smaller the wavelength of the radiation, the greater is the energy carried, i.e. gamma rays are more “energetic” than radio waves. This is shown by the Photoelectric effect in which electrons are ejected from metal surfaces when electromagnetic waves increase so does the speed (and energy) with which electrons are emitted.

Because of its electrical origin, its ability to travel in a vacuum (e.g. from the sun to the earth) and its wave-like properties (i.e. 2 above), electromagnetic radiation is regarded as a progressive transverse wave. The wave is a combination of travelling electric and magnetic fields. The fields vary in value and are directed at right angles to each other and to the direction of travel of the wave.

Figure: 9.75 Electromagnetic wave.

 DETECTION AND APPLICATIONS OF ELECTROMAGNETIC RADIATIONS

The means of detection, the production and application of the components of electromagnetic spectrum are summarised in the following table.

RADIATION

PRODUCTION

DETECTION

APPLICATION

Radio waves

Oscillating electrical circuit

Antennae and aerials

Communications (radio broadcasts, television and satellite communication, cellular telephones, radar and navigation equipment).

Microwaves

Magnetrons and klystrons (special vacuum tubes in microwave oven)

Antennae and aerials

Communication (mobile phones), radar cooking (microwave cookers), speed cameras.

Infrared

Thermal vibration of atoms in hot bodies

Heat sensor, photographic film, semi-conductor devices.

Medical diagnosis (finding hot spots in the body) burglar alarms, military night vision equipment, thermal imaging cameras, and remote controls for TV’s and videos, green houses.

Visible light

Energy level changes of electrons in atoms (anything that is hot enough to glow e.g. the sun).

Eye, photographic film, photo-electric effect, semi-conductor devices fluorescence.

Plant growth, sight, photography, optical fibres and laser beams (used in laser printers, weapon aiming systems, compact disc players).

Ultraviolet

Energy level changes of electrons in atoms (the sun, sparks and mercury vapour lamps)

Photographic film, photoelectric effect, semi-conductor devices, fluorescence.

Killing bacteria, skin treatment, as a source of vitamin D, making ink that fluoresce (for security marking like paper money), make clothes to glow, hardening some types of dental fillings.

x-rays

Bombarding metal with high energy electrons in x-ray tubes, i.e. energy level changes in innermost shells)

Photographic films, ionisation detectors.

Radiography (identifying internal body structures like bones), cancer therapy, crystallography (studying crystal structure), airport security checks.

Gamma rays

Energy changes in the nuclei of radioactive atoms.

Photographic film, ionisation detectors, scintillation counters.

Sterilising food and medical instruments, killing cancer cells and other malignant growths, controlling pests in grains, detecting flaws in metals.

Table 9.7: Production, detection and applications of electromagnetic spectrum

THE EFFECT OF RADIATION ON HUMAN POPULATION

Radiation occurs when unstable nuclei of atoms decay and release particles. There are many different types of radiation. When these particles touch various organic materials such as tissue, damage may, and probably will, be done. Radiation can cause burns, cancers, and death.

 DANGERS OF ELECTROMAGNETIC WAVES

Radio waves: large dose can cause cancer, leukaemia, and other disorders

Microwaves: Prolonged exposure can cause eye defect (cataracts). Recent research indicates that microwaves from mobile phones can affect parts of the brain.

Infrared: can cause overheating

Visible light: too much light can damage the retina in the eye.

Ultraviolet: large doses can damage the retina in the eyes. Large doses can also cause sunburn and skin cancer.

X-rays: Can cause cell damage and cancers.

Gamma rays: can cause cell damage, cancers and mutations in growing tissues.

 CELLPHONES AND ELECTROMAGNETIC RADIATION

Cellphone radiation and health concerns have been raised, especially following the enormous increase in the use of wireless mobile telephony throughout the world. This is because mobile phones use electromagnetic waves in the microwave range. These concerns have induced a large body of research. Concerns about effects on health have also been raised regarding other digital wireless systems, such as data communication networks. The World Health Organisation has officially ruled out adverse health effects from cellular base stations and wireless data networks, and expects to make recommendations about mobile phones in 2007-08.

Cellphone users are recommended to minimise radiation, by for example:

1.       Use hands-free to decrease the radiation to the head.

2.       Keep the mobile phone away from the body.

3.       Do not telephone in a car without an external antenna.

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