The Electromagnetic Spectrum

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Updated February 14, 2004

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The Electromagnetic Spectrum.

Electromagnetic Radiation 

Electromagnetic radiation exists all around us and throughout space. It is produced through the interaction of electric and magnetic fields. These two fields always exist together.

A static (unvarying) electric field will produce a static magnetic field.  A varying electric field produces a varying magnetic field. The reverse is also true. A static magnetic field produces a static electric field. If the magnetic field varies so too will the electric field.

When an electric and magnetic field vary in strength over time they form electromagnetic waves. A flow of electric current creates a corresponding magnetic field. If the current flow is unvarying, the magnetic field will not vary and no electromagnetic waves are produced. A varying, or oscillating, field will produce a changing magnetic field. Together, changing electromagnetic fields produce electromagnetic radiation that travels in waves. This characteristic of electricity and magnetism is used in radios to send information to distant receivers.

Waves are measured by their length (wavelength) and by the frequency with which they pass a point in space (frequency). The length of a wave is usually measured in metres. The frequency is measured by the number of waves or cycles that pass a given point in one second. One cycle per second is called a Hertz (Hz).

Figure 1. 

Say that this set of waves passed by a point in one second. There are 15 waves in this picture. The frequency of this wave is therefore 15 cycles per second (Hz).

Figure 2. 

This set of waves passed by the same point in one second. There are 3 waves in this picture. The frequency of this wave is therefore 3 cycles per second (Hz).

The waves also have different wavelengths. The waves in Figure 1 have a shorter wavelength than in Figure 2. The wavelength of electromagnetic radiation is measured by the distance in metres between the beginning of one wave and the beginning of the next.

The shorter the wavelength the higher the frequency of the radiation. The higher the frequency, the more energy the wave has.

Electromagnetic radiation always travels at the speed of light in a vacuum, or nearly the speed of light if it is traveling through a medium of some kind. The speed of light is approximately 300,000,000 metres per second (300,000 kilometers per second). For practical purposes we can say that radio waves always travel at the speed of light.

While the wavelength and the frequency of radio waves will vary, the speed of light does not. These three aspects of electromagnetic radiation are related to one another and can be described using mathematical formulas. The following formula can be used to determine either the frequency or the wavelength if you know one of the two. 

c = f y

f  = c / y

y = c / f

c = speed of light = 300,000,000 metres per second

f = frequency in Hertz (cycles per second)

y = wavelength in metres

For example, the wavelength (y) of an FM radio station signal with a frequency (f) of 92.5 MegaHertz (92,500,000 Hertz) =  c ( 300,000,000 m per s) / f (92,500,000 Hertz) is 3.24 metres. In other words, 92.5 million waves 3.24 metres long pass your radio each second from that radio station.

The Electromagnetic Spectrum

The electromagnetic spectrum is made up of a family of electromagnetic radiation that includes radio waves, infrared (radiated heat), visible light, ultraviolet, and gamma rays. 

Source: http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html

As you can see from the illustration above, light has a wavelength shorter than the size of a bacterium, radio waves can be as short as a millimeter or many kilometers long.

Our ancient ancestors explored the sky with their unaided eyes using light at optical wavelengths. In the 17th century Galileo turned the newly invented telescope on the Moon and the planets. He discovered craters on the Moon and the fact that there are moons that orbit around Jupiter.  Galileo showed that the investigative power of the human eye and brain could be extended by using technology. 

Light represents a surprisingly narrow range of wavelengths within the electromagnetic spectrum. Yet with light we have discovered a great deal about the universe. Even more can be learned if we observe the Universe at other wavelengths.

Humans have evolved on the surface of the Earth, bound there, until recently, by the force of gravity. We are naturally able to use our eyes to observe the world around us at optical wavelengths. Above and around us is an atmosphere that provides us with oxygen and rainwater, and that protects us from radiation from the Sun and from space. At the same time, it prevents certain wavelengths of electromagnetic radiation from reaching the surface of the Earth where we can observe them with our technology. This is fortunate for us since X-rays, Gamma rays, and ultra-violet radiation would destroy life on our planet without the protective layer of atmosphere that surrounds it.

Light and radio waves up to a certain frequency can penetrate the Earth's atmosphere. The rest cannot reach the surface of the planet. See the illustration below.

Source: http://www.drao.nrc.ca/outreach/probing/

Radio Astronomy

It was not until the 20th century that we discovered that radio waves can serve as another window on the universe. In 1932 an American radio engineer named Karl Jansky discovered that radio emissions causing interference on radio broadcasts were coming from space. The most powerful of these were coming from the constellation Sagitarius in the direction of the centre of our galaxy. At the time and until recently, this source went unexplained. We are now fairly certain the source of some of this radio energy is a huge and powerful black hole at the galaxy's centre. 

In 1937 another American named Grote Reber, an amateur, built a parabolic dish antenna in his backyard and in 1938 confirmed Jansky's discovery. The second World War disrupted any further application of this discovery to the exploration of space.

In spite of this, radar operators in Britain during the war observed interference on their radar sets that was ultimately discovered to be radio emissions from the Sun. After the war, this discovery and the work of Jansky and Reber came to the attention of the broader scientific community and the science of radio astronomy was born.

A Canadian, A.E. Covington of the National Research Council in Ottawa, was one of the first to use the new technology of radio astronomy to learn about radio emissions from the Sun. In 1946 he used old World War II radar equipment to begin monitoring the Sun. He carried out daily measurements of the total radio output of the Sun at a wavelength of 10.7 cm that have continued to the present day and form a benchmark reference for radio astronomers around the world.

The Radio Spectrum

The radio spectrum extends from the very long (up to  30,000 kilometres in length) to short (millimetres long). Radio waves are very long compared to the rest of the electromagnetic spectrum. The radio spectrum is divided up into a number of "bands" based on their wavelength and usability for communication purposes. They extend from the Very Low Frequency portion of the spectrum through the Low, Medium, High, Very High, Ultra High, and Super High to the Extra High Frequency range as depicted in the illustration below. Above the EHF band comes infrared radiation and then visible light.

The illustration above also shows where commonly used radio devices operate in the radio spectrum. The table below gives detailed information about the frequency range of each band, the length of waves in each band and some typical uses of that portion of the radio spectrum. Each band and subsections of each band have characteristics  that make them suitable for particular uses. Note that at the lower frequencies typical uses tend to be long range and marine applications, while at very short wavelengths the uses tend to be space based. 

Band Frequency Wavelength Some Uses
VLF 3 - 30 kHz 100 km - 10 km Long range navigation and marine radio
LF 30 - 300 kHz 10 km - 1 km Aeronautical and marine navigation
MF 300 kHz - 3 MHz 1 km - 100 m AM radio  and radio telecommunication
HF 3 - 30 MHz 100 m - 10 m Amateur radio bands, NRC time signal
VHF 30 - 300 MHz 10 m - 1 m TV, FM, cordless phones, air traffic control
UHF 300 MHz - 3 GHz 1 m - 10 cm UHF TV, satellite, air traffic radar, etc
SHF 3 - 30 GHz 10 cm - 1 cm Mostly satellite TV and other satellites
EHF 30 - 300 GHz 1 cm - 1 mm Remote sensing and other satellites
 

 

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and the

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