spectrum - Modern meaning in the physical sciences

The distribution of electromagnetic energy as a function of wavelength or frequency. A common example is the spectrum of white light dispersed by a prism to produce a rainbow of constituent colours; the rainbow is the spectrum of sunlight refracted through raindrops. For white light the colours are red, orange, yellow, green, blue, and violet in order of decreasing wavelength. All objects with temperatures above absolute zero emit electromagnetic radiation by virtue of their warmth alone; this black body radiation is emitted at progressively shorter wavelengths as temperature is increased. The universe (3K) sends us microwaves; objects at room temperature (295K) infrared rays; the Sun (6000K) yellow light; and the solar corona (1 million K) X-rays. Individual atoms can emit and absorb radiation only at particular wavelengths corresponding to the transitions between energy levels in the atom. The spectrum of a given atom (or element) therefore consists of a series of emission or absorption lines. A familiar example is sodium, used in low pressure street lights, where almost all the energy emerges as a pair of lines in the yellow part of the spectrum. The device for displaying a spectrum is called a spectrograph. By analysing the spectrum of a substance, its chemical composition can be deduced.

A spectrum (plural: spectra) is a condition or value that is not limited to a specific set of values but can vary infinitely within a continuum. The word has evolved from the old English word spectre meaning a ghost or apparition, but the modern meaning now comes from its use within science.

Spectrum saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; Thus one might talk about the spectrum of political opinion, or the spectrum of activity of a drug, or the autistic spectrum.

In most modern usages of spectrum there is a unifying theme between extremes at either end.

Modern meaning in the physical sciences

In the 17th century the word spectrum was introduced into optics, referring to the range of colors observed when white light was dispersed through a prism. Soon the term referred to a plot of light intensity as a function of frequency or wavelength.

The term spectrum was soon applied to other waves, such as sound waves, and now applies to any signal that can be decomposed into frequency components. Sometimes, the word spectrum refers to the compound signal itself, such as the "spectrum of visible light", a reference to those electromagnetic waves which are visible to the human eye. Looking at light through a prism separates visible light into its colors according to wavelength. Violet at one end has the shortest wavelength and red at the other end has the longest wavelength of visible light. As the wavelengths get bigger below the red visible light they become infrared, microwave, and radio. As the wavelengths get smaller above violet light, they become ultra-violet, x-ray, and gamma ray.