Spectrum (plural: Spectra), the distribution of radiant energy arranged by wavelength, frequency, or some other measurable order. The electromagnetic spectrum includes the entire range of radiant energy from the shortest gamma rays to the longest electric waves. Certain parts of the electromagnetic spectrum are often considered to be separate, less extensive spectra. Among these are the spectra of radio waves, X rays, and visible light, or color.

Perhaps the most familiar spectrum is that of visible light. It occurs in nature when sunlight passes through raindrops to form a natural spectrum known as a rainbow. Although occupying only a small part of the electromagnetic spectrum, the visible light spectrum includes all wavelengths of energy visible to the normal human eye. These wavelengths make up the full range of colors from violet through red. Reflected light and light from a heated, glowing source often contains several of the colors. By passing the light through an instrument called a spectroscope, the light is dispersed, or separated, into its individual wavelengths, each of which makes up one color. The spectroscope acts in the same way the raindrops do when they break up sunlight to form the rainbow.

A spectrum may be either continuous or discontinuous. A continuous spectrum spreads out in the spectroscope to form an unbroken, rainbowlike band of colors from violet through red. In a discontinuous spectrum the band is broken by colorless gaps or by dark lines called Fraunhofer lines.

Spectra are also grouped according to how the light is measured within the spectroscope. An emission spectrum originates from light admitted to the spectroscope directly from the substance being studied. An absorption spectrum is formed when a relatively cool gas (or liquid or solid) is placed in the path of the light entering the spectroscope. The gas absorbs certain wavelengths of the light, creating a discontinuous spectrum. Absorption spectra are the types most often used in chemical and astronomical studies.