| Contents for this page | Related topics |  |
|---|---|---|
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Introduction The wave nature of EM radiation The speed of EM radiation The particle nature of EM radiation The electromagnetic spectrum |
 Data Glossary | |
| Learning Outcomes | ||
| After studying this section, you will be familiar with (a) the dual nature of electromagnetic radiation, (b) the speed of electromagnetic radiation, and (c), the extent and range of the electromagnetic spectrum. | ||
| Helpful background knowledge | ||
| The photoelectric effect | ||
Electromagnetic radiation is an integral part of the universe, and surround us constantly. The mere acts of seeing the objects around us, watching TV, listening to the radio, using a mobile phone or a microwave oven involve the presence of electromagnetic radiation. Electromagnetic radiation was not properly understood, if at all, until Maxwell laid its mathematical foundations. Further work by Hertz, Planck, Einstein and de Broglie has given us the theoretical framework on which a huge variety of technological marvels are based.
Electromagnetic radiation is due to the mutual induction of oscillating electric and magnetic fields that change in value with time, and travel in a wave form (ELECTROMAGNETIC WAVES) through space at a speed (in a vacuum) of 2.99792458x108 m.s-1 (
). In the animated diagram above, the electric field is shown in red, and the magnetic field in blue. As the electric field increases, it induces an increasing magnetic field at right angles to it, and vice-versa. The waves, in the absence of any intervening absorbing medium, propagate indefinitely. Astronomers can detect light from distant astronomical objects that has travelled for several billion years!
If we consider any plane perpendicular to the direction of travel, the values of the electric and magnetic fields at any time can be represented as vectors at 90º to each other, whose magnitude and direction change with time as shown in the animation (the red vector changes its direction from "up" to "down", while the blue vector changes its direction from right to left.)
Being a wave, the radiation will be characterised by a wavelength, l, a frequency, f, where f = c/l and c the speed of electromagnetic radiation (light).
The speed of electromagnetic waves in a vacuum, commonly referred to as the SPEED OF LIGHT, with symbol c, is a fundamental constant, independent of time, place and the reference frame of the observer (). In a vacuum, c has the EXACT value of 2.99792458x108 m.s-1 (), and there is as yet no generally accepted evidence to support the notion that this value has ever changed with time. Values obtained prior to the 1950's are only of historical interest due to the lack of precision obtainable with the equipment available in those days. The speed of light was first determined experimentally in 1675 by Roemer and Picard, who obtained a value of 2.9927x108 m.-1, with an accuracy of plus or minus 5%.
In 1983, the International Conference on Weights and Measures adopted the value c = 299 792 458 m.s-1, thus making the value for the speed of light a defined fundamental physical constant.
After studying the photoelectric effect, Einstein concluded that electromagnetic radiation could be shown to behave as a beam of particles. These particles are called PHOTONS, and have no mass.
Photons do however have energy, which is related to the frequency or wavelength of the radiation according to Planck's law
which enables us to calculate the energy of a photon of any given wavelength.
Since the amount of energy per particle is very small, physicists frequently use units of ELECTRON-VOLT, eV, where 1 eV = 1.60x10-19 J. The smaller the wavelength, that is, the greater the frequency, the more energy will a particle have. High energy radiation has a high PENETRATING ABILITY and causes physical damage to living cells. An example of this are X-rays and the gamma-rays that arise from the decay fo certain radioactive isotopes. This is discussed more fully in the Grade 11 topic on radioisotopes.
Electromagnetic waves spanning the frequency range from 10 Hz to 1025 Hz have been discovered. Different parts of the spectrum are given different names, for example: radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays.
You can see from the above diagram that the region to which our eyes are attuned, the visible spectrum, occupies but a small part of the overall spectrum.