The Electromagnetic Spectrum
Introduction


For most of the nineteenth century, optics, magnetism, and electricity were seen as very different fields of study, even though the famous scientists of the time were working in all of these as well as other areas.



The nature of light is something that has intrigued scientists for a long time. By about 1830, most scientists had accepted the wave theory of light and by the start of the twentieth century it was considered an electromagnetic wave. In the early 1900s light was shown to have a particulate nature (photons) as well and is now widely considered as exhibiting wave–particle duality.

One of the greatest scientific advances of the nineteenth century was made by the Scottish physicist James Clerk Maxwell, who produced a single theory explaining experimental observations in electricity, magnetism, and light. Maxwell's theory explained the observations of Ørsted (also spelled Oersted), Faraday, and others and also went on to predict that electromagnetic fields could travel through space. It was not until 1887, eight years after Maxwell's death, that Henrich Hertz generated and detected the first of what we now call electromagnetic waves.

James Clerk Maxwell
Faraday introduced the concept of a field to explain how an electrically charged object or a magnet could affect similar objects even when the objects were not touching.
Figure 1.   James Clerk Maxwell 1831–79
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Faraday's ideas were not widely accepted until Maxwell showed that all electric and magnetic phenomena could be described using only four equations based on electric and magnetic fields. The four equations are called Maxwell's equations and are as important to the study of
electromagnetic
Electromagnetic waves, such as light, are made up from oscillating electric and magnetic fields. Because of this, they are self-propagating and can travel through a vacuum. All types of electromagnetic wave travel at the same speed in a vacuum, 3 × 108 ms−1.
electromagnetic
waves as Newton's famous equations are in the study of mechanics.

Maxwell used his equations to calculate the speed at which electromagnetic (EM) waves should travel through a vacuum. The value he calculated was 3 × 108 ms−1. The similarity of this value to the speed of light, which had been measured earlier, led Maxwell to conclude that light is a type of electromagnetic wave.

The wave nature of light had been known for some time, but the source of the oscillations in the light wave was not understood. Maxwell considered electromagnetic waves, such as light, to be made up from oscillating electric and magnetic fields. Maxwell's theory links the speed of electromagnetic waves through any material with other fundamental electric and magnetic constants.

Figure 2.   Representation of an EM wave.
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Following Maxwell's work, the search for other electromagnetic waves was taken up and in 1887 waves having the same speed as light but with wavelengths of a few metres were discovered. The experimental work on the discovery of these radio waves was done by Henrich Hertz. Throughout the century following Maxwell's work other types of radiation have been detected which can be classed as electromagnetic.

To be classed as electromagnetic, waves must …

  • be transverse;

  • travel at the same speed (3 × 108 ms−1) in a vacuum;

  • and be unaffected by external magnetic or electric fields.



Although all members of the electromagnetic spectrum travel at the same speed in a vacuum, their frequencies and wavelengths differ. It is useful to remember that the speed,
frequency
The wave frequency f is the number of complete waves passing any point each second. Frequency is measured in hertz, Hz.
frequency
, and
wavelength
The wavelength is the distance from one point on a wave to the identical point on the next wave. This can be stated as the distance from a crest on a wave to the next crest on the wave.
wavelength
of any type of wave are connected by the equation v = f. The speed of light through a vacuum is given the symbol, c. so for all electromagnetic waves travelling through a vacuum we can say:

Members of the electromagnetic spectrum
Generally the different types of electromagnetic waves are classified according to their wavelength, and their frequencies are determined from the equation c = f.

Figure 3.   Properties of different EM waves.
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Complete the following statements for one type of wave.

  • The members of the electromagnetic spectrum with the longest wavelengths are called waves. They can pass through but are absorbed by objects. Some are by ions in the upper atmosphere.
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Complete the following statements for another type of wave.

  • EM radiation with wavelengths between 1 m and 1 mm is called . This is used widely in . They can pass through but are absorbed by . This means that they can be used in food.
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Hot objects radiate which of the following types of radiation?
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Which type of radiation carries lots of
energy
A system has energy when it has the capacity to do work. The scientific unit of energy is the joule.
energy
and can penetrate several cm of lead shielding?
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Microwaves
Nowadays we take communication for granted. We chat on the phone to distant friends. We use the Internet to get information from computers anywhere on Earth. We can listen to the radio or watch TV when we go away on holiday. But it was only a century ago that the Marconi brothers started experimenting with radio, and Alexander Bell had the first telephone conversation just 25 years before then. Imagine what life would be like today without these inventions. Click any of the three circles in Fig.4 to see the different ways that data can arrive in our homes.

Figure 4.   EM waves and communications.
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Radiation in the microwave region of the EM spectrum is widely used in communications because it is fairly directional and can penetrate the Earth's atmosphere.

When the frequency of the
microwaves
Microwaves are electromagnetic waves with wavelengths in the range 1 mm to 0.1 m.
microwaves
matches the
natural frequency
When an oscillating system is allowed to vibrate freely, it tends to do so at a characteristic frequency determined by certain parameters of the system itself (e.g. the mass and spring constant of a loaded spring). This frequency is known as the natural frequency.
natural frequency
of the bonds in a water molecule,
resonance
Resonance occurs when a vibrating system is driven at its natural frequency by an external source of energy. When this happens the amplitude of the system rapidly reaches its maximum possible value.
resonance
occurs and energy is transferred to shake the water molecules. This raises the temperature of the water and so
microwaves
Microwaves are electromagnetic waves with wavelengths in the range 1 mm to 0.1 m.
microwaves
can be used to re-heat or cook food.

Infrared radiation
All objects at temperatures above
absolute zero
Absolute zero is the lowest possible temperature: −273 °C. At this temperature all particles stop moving, and gases exert no pressure at all.
absolute zero
give off
infrared
Infrared radiation is electromagnetic radiation in the wavelength range 700 nm to 1 mm. It is produced by hot objects.
infrared
radiation (IR), but hot objects give off more than cold ones. All objects also absorb IR radiation. When you feel the warmth of a heater or the sun, your skin is absorbing the IR radiation. Fig.5 shows an aerial photograph taken of a forest fire in California. Click on the picture to switch between a
normal
The normal to a surface at a given point is a line drawn at right angles to the surface at that point.
normal
photograph and an IR photograph.

Figure 5.   Forest fire in visible light and infrared.
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The hottest part of the fire shows up as bright white in the IR picture. In the visible photograph, the centre of the fire is hidden by the smoke so an IR photograph can be used to find the centre of the blaze.

Infrared detectors are also used in night vision goggles which have a number of military and rescue services applications. Fig 6 shows view of an unlit street on a dark night! There are no sources of light so nothing can be seen. Move the pointer into the picture. Click and hold while moving the pointer around get the view that would be observed through an infrared camera.

Figure 6.   Using infrared for night vision.
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The infrared camera detects radiation emitted by the objects in the picture and turns this into visible light which our eyes can detect.

The greenhouse effect
The sun emits EM radiation of all wavelengths. When infrared radiation enters a greenhouse the plants inside warm up. The warm plants also emit infrared radiation, however since their temperature is lower than that of the sun the infrared emitted by the plant has a longer wavelength. This longer wavelength radiation is unable to penetrate the greenhouse glass and is reflected back inside. In this way the radiation gets trapped and the greenhouse warms.
Figure 7.   The greenhouse effect.
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Some gases in the atmosphere, such as carbon dioxide and methane, behave like the glass in the greenhouse. They trap the long wavelength IR radiation and keep the Earth warm enough to live on. These gases occur naturally, but they are also produced as exhaust gases by cars,
power
The power of system is a measurement of the rate at which energy is transferred from one form to another. The scientific unit of power is the watt.
power
stations, and factories. As we produce more of these gases, more radiation is trapped by the atmosphere.

A growing number of scientists fear that the earth may experience global warming as a result of this increase in the greenhouse effect. This could have very serious consequences including melting of the polar ice caps, a rise in sea levels, and widespread climatic change.

Visible radiation
As with all parts of the electromagnetic spectrum, visible light has a range of wavelengths. What makes the visible part of the spectrum special is that we can detect its wavelengths with our eyes!
Click on the figure below to interact with the model.

 Figure 8.  Move the prism into the beam and note the spectrum of colours produced.



When white light is dispersed, we can see the constituent colours of the range of visible wavelengths.


Colour Typical wavelength / nm
Red 640
Yellow 590
Green 510
Blue 480



A beam of white light passing through a prism will be dispersed and, if a suitable source is used, it may be possible to detect some UV radiation near the blue end of the spectrum of colours and perhaps a little infrared just beyond the red end.

Ultraviolet
The sun emits radiation over the whole EM spectrum, from short wavelength gamma radiation to long wavelength radio waves. However, most of the energy the sun radiates is in the
ultraviolet
Ultraviolet radiation is electromagnetic radiation in the wavelength range 13 nm to 380 nm.
ultraviolet
(UV), visible, and infrared portions of the spectrum. We feel the IR waves as warmth.

In moderation, UV rays darken the skin causing a tan.

Figure 9.   Absorption of ultraviolet by skin.
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Complete the following statements summarizing the interaction between the sun and tissue.

  • If you are exposed to light for a long time or if the light is very bright the rays will penetrate the and damage the cells in the tissue underneath. skin will be damaged more than dark skin because it has less . The melanin in dark skin UV light passing through and so gives increased to the flesh underneath.



    Similarly, applying suntan lotion places a in the way and prevents the UV rays from the sun damaging the tissue underneath.
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Sunbeds contain UV lamps and are used to give you a tan. However, overuse of a sunbed can lead to premature ageing and sagging of your skin as well as increasing the risk of developing skin cancer.

X-rays
Since their discovery by Röntgen in 1895,
X-rays
X-rays are high-energy electromagnetic waves produced by directing a beam of high-energy electrons at a metal target.
X-rays
have been used extensively in medicine. They are produced when very fast moving electrons are rapidly decelerated by striking targets of tungsten or molybdenum. The emission of radiation as a result of the rapid
deceleration
The deceleration of an objects is a measurement of its rate of change of velocity as it slows down.
deceleration
of the electrons is called bremsstrahlung (from a German word meaning 'braking radiation'). Only a very small part of the electron's
kinetic energy
The kinetic energy of a system is a measurement of the energy associated with its translational motion.
kinetic energy
is transferred into X-rays.

Figure 10.   Production of X-rays.
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Approximately 99.5 per cent of their kinetic energy goes to heating the target, which must be cooled. The X-rays emerge through the window as a narrowly focused beam. This beam can be used for many applications, particularly in medicine.

Figure 11.   Drag the box over the hand to get an X-ray picture.
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The use of X-rays to show breaks in bones demonstrates that their penetration of matter depends upon the density of the material through which they are passing. Penetration is lowest for dense materials such as bone.

X-rays also affect photographic film. This is a convenient way of showing the location of breaks in bones or of locating imperfections in welded metal joints and castings. A source of X-rays is placed inside the joint to be tested and photographic film wrapped around the outside. Cracks or weak spots will allow more of the radiation to pass through the joint. When the photographic film is developed the position of a crack can be established. The short wavelength of X-rays also makes them a useful tool for probing the structures of certain crystals or organic molecules.

Summary


The sun emits radiation over the entire EM spectrum, from short wavelength gamma radiation to long wavelength radio waves.

All EM waves are transverse and will travel through a vacuum at the same speed.

The properties and applications of different parts of the spectrum depend upon their frequency and wavelength.

Exercises
1. Match the following types of electromagnetic radiation to the appropriate range of wavelengths.
  • Infrared
    Microwaves
    Ultraviolet
    Visible light
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2. Which of the following properties are shown by all forms of electromagnetic radiation?
  • They are not affected by external electric or magnetic fields.
    They are given off by hot objects.
    They can be detected by photographic film.
    They travel as transverse waves.
    They travel at 3 × 108 ms−1 in a vacuum.
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