Spectrum

Light can be pictured as traveling in the form of a wave. A wave is a series of regularly spaced peaks and troughs. The distance between adjacent peaks (or troughs) is the wavelength, symbolized by the Greek letter lambda ( λ). For a light wave traveling at a speed, c, the number of peaks (or troughs) which pass a stationary point each second is the frequency of the wave, symbolized by the Greek letter nu ( ν). The units of frequency are number per second, termed Hertz (Hz). The frequency of a wave is related to the wavelength and the speed of the wave by the simple relation: ν = c/ λ. The speed of light depends on the medium through which it is passing, but, as light travels primarily only through air or space, its speed may be considered to be constant, with a value of 3.0 × 10⁸ meters/sec. Therefore, since c is a constant, light waves may be described by either their frequency or their wavelength, which can be interconverted through the relation ν = c/ λ.

Interestingly, Newton did not think light traveled as a wave, but rather he believed light to be a stream of particles, which he termed corpuscles, emitted by the light source and seen when they physically entered the eye. It was Newton's contemporary, the Dutch astronomer Christiaan Huygens (1629-1695), who first theorized that light traveled from the source as a series of waves. In the quantum mechanical description of light, the basic tenets of which were developed in the early 1900s by Figure 3. Schematic of the absorption spectrum of atomic hydrogen recorded on a photographic plate. Illustration by Hans & Cassidy. Courtesy of Gale Group. Max Planck and Albert Einstein, light is considered to possess both particle and wave characteristics. A "particle" of light is called a photon, and can be thought of as a bundle of energy emitted by the light source. The energy carried by a photon of light, E, is equal to the frequency of the light, ν , multiplied by a constant: E = h ν, where h is Planck's constant, (h = 6.626 × 10^-34 joulesseconds), named in honor of Max Planck. Thus, according to the quantum mechanical theory of light, light traveling through air or space may be described by any one of three inter-related quantities: frequency, wavelength, or energy. A spectrum of light may therefore be represented as a distribution of intensity as a function of any (or all) of these measurable quantities.