SPECTRUM

A spectrum is a plot or distribution of a physical quantity across a continuum of wavelengths, frequencies, or energies. In common usage it most often refers to electromagnetic radiation, but the term is also used for sound and other domains.

In optics, spectra describe how light is distributed across wavelengths. A continuous spectrum contains light at all wavelengths within a range, as produced by a hot dense object. A line spectrum shows light at discrete wavelengths emitted or absorbed by atoms or molecules.

Emission spectra arise when excited atoms emit photons; absorption spectra occur when light passes through a medium and specific wavelengths are absorbed, creating dark lines in the spectrum. The pattern of lines acts as a fingerprint for the substance.

The electromagnetic spectrum spans radio to gamma rays, categorized roughly as radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma-ray. The visible region corresponds to roughly 380 to 750 nanometers.

Spectrometry and spectroscopy are techniques for measuring spectra with instruments called spectrometers, spectrographs, or prisms, extracting information about composition, temperature, motion, and physical conditions.

In astronomy, spectra reveal chemical composition and redshift; in chemistry, spectra identify substances; in industry, spectral analysis monitors quality.

The term also appears in other fields to denote a continuum of values across a scale.