Spectrum
This article deals with the general meaning of spectrum and the history of its use. For other meanings and specific uses within the physical, mathematical, and biological sciences, see spectrum (disambiguation).A
spectrum (plural:
spectra) is a condition or value that is not limited to a specific set of values but can vary infinely 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; it has since been applied by analogy to many fields. Thus one might talk about the spectrum of
political opinion, or the
spectrum of activity of a drug, or the
autistic spectrum. In these uses, values within a spectrum are not necessarily precisely defined numbers as in optics; exact values within the spectrum are not precisely quantifiable. Such use implies a broad range of conditions or behaviors grouped together and studied under a single title for ease of discussion.
In most modern usages of
spectrum there is a unifying theme between extremes at either end. Some older usages of the word did not have a unifying theme, but they led to modern ones through a sequence of events set out below. Modern usages in mathematics did evolve from a unifying theme, but this may be difficult to recognize.
Originally a
spectrum was what is now called a
spectre, i.e., a
phantom or
apparition.
Spectral evidence is testimony about what was done by spectres of persons not present physically, or
hearsay evidence about what ghosts or apparitions of Satan said. It was used to convict a number of persons of
witchcraft at
Salem, Massachusetts in the late
17th century.
 |
High-resolution spectrum of the Sun showing thousands of elemental absorption lines (Fraunhofer lines). |
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. A
spectrum is a usually
2-dimensional plot, of a compound signal, depicting the components by another measure. 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. The colors in order are violet, blue, green, yellow, orange, red. 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.
*
Electromagnetic spectrum*
Frequency*
Emission Spectrum Photos of Various Light Sources
