The term electromagnetic spectrum refers to all forms of energy transmitted by means of waves traveling at the speed of light. Visible light is a form of electromagnetic radiation, but the term also applies to cosmic rays, X rays, ultraviolet radiation, infrared radiation, radio waves, radar, and microwaves. These forms of electromagnetic radiation make up the electromagnetic spectrum much as the various colors of light make up the visible spectrum (the rainbow).
Any wave—including an electromagnetic wave—can be described by two properties: its wavelength and frequency. The wavelength of a wave is the distance between two successive identical parts of the wave, as between two wave peaks or crests. The Greek letter lambda (λ) is often used to represent wavelength. Wavelength is measured in various units, depending on the kind of wave being discussed. For visible light, for example, wavelength is often expressed in nanometers (billionths of a meter); for radio waves, wavelengths are usually expressed in centimeters or meters.
Frequency is the rate at which waves pass a given point. The frequency of an X-ray beam, for example, might be expressed as 10 18 hertz. The term hertz (abbreviation: Hz) is a measure of the number of waves that pass a given point per second of time. If you could watch the X-ray beam from some given position, you would see 1,000,000,000,000,000,000 (that is, 10 18 ) wave crests pass you every second.
For every electromagnetic wave, the product of the wavelength and frequency equals a constant, the speed of light (c). In other words, λ · f = c. This equation shows that wavelength and frequency have a reciprocal relationship to each other. As one increases, the other must decrease. Gamma rays, for example, have very small wavelengths and very large frequencies. Radio waves, by contrast, have large wavelengths and very small frequencies.
As shown in the accompanying figure, the whole range of the electromagnetic spectrum can be divided up into various regions based on wavelength and frequency. Electromagnetic radiation with very short wavelengths and high frequencies fall into the cosmic ray/gamma ray/ultraviolet radiation region. At the other end of the spectrum are the long wavelength, low frequency forms of radiation: radio, radar, and microwaves. In the middle of the range is visible light.
Properties of waves in different regions of the spectrum are commonly described by different notation. Visible radiation is usually described by its wavelength, while X rays are described by their energy. All of these schemes are equivalent, however; they are just different ways of describing the same properties.
Electromagnetic radiation: Radiation that travels through a vacuum with the speed of light and that has properties of both an electric and magnetic wave.
Frequency: The number of waves that pass a given point in a given period of time.
Hertz: The unit of frequency; a measure of the number of waves that pass a given point per second of time.
Wavelength: The distance between two successive peaks or crests in a wave.
The boundaries between types of electromagnetic radiation are rather loose. Thus, a wave with a frequency of 8 × 10 14 hertz could be described as a form of very deep violet visible light or as a form of ultraviolet radiation.
The various forms of electromagnetic radiation are used everywhere in the world around us. Radio waves are familiar to us because of their use in communications. The standard AM radio band includes radiation in the 540 to 1650 kilohertz (thousands of hertz) range. The FM band includes the 88 to 108 megahertz (millions of hertz) range. This region also includes shortwave radio transmissions and television broadcasts.
Microwaves are probably most familiar to people because of microwave ovens. In a microwave oven, food is heated when microwaves excite water molecules contained within foods (and the molecules' motion produces heat). In astronomy, emission of radiation at a wavelength of 8 inches (21 centimeters) has been used to identify neutral hydrogen throughout the galaxy. Radar is also included in this region.
The infrared region of the spectrum is best known to us because of the fact that heat is a form of infrared radiation. But the visible wavelength range is the range of frequencies with which we are most familiar. These are the wavelengths to which the human eye is sensitive and which most easily pass through Earth's atmosphere. This region is further broken down into the familiar colors of the rainbow, also known as the visible spectrum.
The ultraviolet range lies at wavelengths just short of the visible range. Most of the ultraviolet radiation reaching Earth in sunlight is absorbed in the upper atmosphere. Ozone, a form of oxygen, has the ability to trap ultraviolet radiation and prevent it from reaching Earth. This fact is important since ultraviolet radiation can cause a number of problems for both plants and animals. The depletion of the ozone layer during the 1970s and 1980s was a matter of some concern to scientists because of the increase in dangerous ultraviolet radiation reaching Earth.
We are most familiar with X rays because of their uses in medicine. X-radiation can pass through soft tissue in the body, allowing doctors to examine bones and teeth from the outside. Since X rays do not penetrate Earth's atmosphere, astronomers must place X-ray telescopes in space.
Gamma rays are the most energetic of all electromagnetic radiation, and we have little experience with them in everyday life. They are produced by nuclear processes—during radioactive decay (in which an element gives off energy by the disintegration of its nucleus) or in nuclear reactions in stars or in space.