Electromagnetic Radiation: Radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays are all manifestations of the same thing: electromagnetic radiation. The difference between the various types is a consequence of the difference in frequency (or wavelength). The theory was worked out in the 19th century by James Clark-Maxwell; who showed that electricity and magnetism are related and that light consists of electric and magnetic fields linked together as they travel through space. Maxwell's work set the stage for the development of radio and countless other technologies which we nowadays take for granted. The relationship between frequency and wavelength is: c=fl, Where c is the speed of light (299 792 458 metres per second), f is the frequency, and l (the Greek letter lambda, lower case) is the wavelength, ie., the distance the wave travels as it undergoes one complete cycle. The middle of the medium-wave broadcast band is about 1MHz (one million cycles per second) and the corresponding wavelength is 300 metres. The wavelength at the middle of the visible spectrum is about 500nm (nanometres = thousand millionths of a metre), and the corresponding frequency is 6 x 10^14Hz, ie., 600 000 000 000 000 Hz (600THz or 600 Tera Hertz). Radio signals are normally characterised by their frequency, but light is normally characterised by its wavelength because the numbers for frequency are messy to write down without using scientific notation. Electromagnetic radiation is also sometimes split into two classes: ionising radiation, ie., radiation which has sufficient energy to knock electrons out of atoms and molecules, and non-ionising radiation, which hasn't. Radio and infrared are classed as non-ionising radiations (although some things can be ionised by infrared), whereas visible light, UV, X-rays and gamma rays are definitely ionising radiations. Our body senses are divided according to this distinction; in that we sense infrared through the skin by its heating effect, whereas we sense visible light through our eyes by its ability to split molecules of light-sensitive pigment. Insects can also see in the UV part of the spectrum, whereas to the human body, anything of shorter wavelength than visible light is simply harmful.

Return to glossary