The term spectrum has two different, but closely related, meanings. In general, the term refers to a whole range of things. In everyday life, for example, a person might say that he or she is interested in the whole spectrum of news stories, meaning that he or she enjoys reading and hearing about anything to do with the news.
In the field of science, one meaning for the word spectrum has to do with the whole range of electromagnetic energies that exist. This range is known as the electromagnetic spectrum. All forms of electromagnetic energy travel through space in the form of waves that have distinctive wavelengths and frequencies. The wavelength of a wave is the distance between adjacent identical parts of the wave, as between two crests or two troughs (pronounced trawfs). The frequency of a wave is the number of crests (or troughs) that pass a given point in space per second.
The electromagnetic spectrum consists of forms of energy such as gamma rays, X rays, ultraviolet radiation, infrared radiation, visible light, radio waves, microwaves, and radar. These forms of energy are similar in their mode of transmission but different from each other in their wavelength and frequency.
Absorption spectrum: The spectrum formed when light passes through a cool gas.
Continuous spectrum: A spectrum that consists of every possible wavelength of light or energy.
Electromagnetic spectrum: The continuous distribution of all electromagnetic radiation with wavelengths ranging from approximately 10 −15 to 10 6 meters, which includes gamma rays, X rays, ultraviolet, visible light, infrared, microwaves, and radio waves.
Emission spectrum: The spectrum produced when atoms are excited and give off energy.
Frequency: For a wave, the number of crests (or troughs) that pass a stationary point per second.
Line spectrum: A spectrum that consists of a few discrete lines.
Wavelength: The distance between adjacent peaks (peaks located next to each other) or troughs on a wave.
The term spectrum is also used in describing the whole range of visible light, ranging from red through orange, yellow, green, and blue to violet. If all colors are represented in the spectrum, it is called a continuous spectrum. A rainbow is an example of a continuous spectrum.
When any one given element is heated, it also gives off a spectrum—but one that is not continuous. Instead, it gives off a series of lines that reflect specific electron changes that occur within the atoms of that element. Some elements have very simple line spectra consisting of only a handful of lines. Other elements give off more complex line spectra with many lines.
Line spectra can take on one of two general forms: emission or absorption spectra. An emission spectrum is the line pattern formed when an element is excited and gives off energy. An absorption spectrum is formed when white light passes through a cool gas. The gas absorbs certain wavelengths of energy and allows others to pass through. The line spectrum formed by the energy that passes through the gas is known as an absorption spectrum.