spectra

Spectra is the plural form of spectrum, used across science to denote either a distribution of a physical quantity as a function of wavelength or frequency, or, in mathematics, a set of values associated with an operator. In everyday scientific language, spectra most often refer to electromagnetic radiation or light, but the term also applies to other domains such as acoustics and signal processing.

In the context of electromagnetic radiation, the electromagnetic spectrum spans from radio waves to gamma rays. A spectrum can be continuous, showing a smooth spread of wavelengths; an emission spectrum, displaying bright lines at specific wavelengths when a material emits light; or an absorption spectrum, showing dark lines where light has been absorbed by matter. Spectra are typically produced and analyzed using spectroscopy, the study of how matter interacts with electromagnetic radiation.

Spectra are fundamental for identifying materials and measuring their properties. By examining spectral lines and features, scientists infer chemical composition, temperature, density, and motion. Doppler shifts in spectral lines reveal radial velocities; shifts toward longer wavelengths indicate recession, while shifts toward shorter wavelengths indicate approach.

In astronomy, spectra from stars, galaxies, and other objects provide crucial information about composition, temperature, age, and distance. They enable measurements of speeds, rotations, magnetic fields, and redshifts that inform models of cosmic expansion and structure.

In mathematics, the spectrum of an operator is the set of scalars for which the operator fails to be invertible. This spectrum divides into the point spectrum (eigenvalues), the continuous spectrum, and the residual spectrum, and it plays a central role in functional analysis and related fields. Spectra also appear in acoustics and signal processing as representations of frequency content.