Everything about Spectrum totally explained
A
spectrum (plural
spectra or
spectrums) is a condition that isn't limited to a specific set of values but can vary infinitely within a
continuum. The word 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
autism 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's a unifying theme between extremes at either end. Some older usages of the word didn't 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.
Origins
In
Latin spectrum means "image" or "
apparition", including the meaning "
spectre".
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.
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 or
power as a function of
frequency or
wavelength, also known as a
spectral density.
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. It separates them according to its dispersion relation and a grating separates according to the grating equation and if massive particles are measured often their speed is measured. To get a spectrum, the measured function has to be transformed in their independent variable to frequencies and the dependent variable has to be reduced in regions, where the independent variable is stretched. For this imagine that the spectrum of pulse with a finite number of particles is measured on a film or a CCD. Assuming no particles are lost, any nonlinearity (compared to frequency) on the spectral separation concentrates particles at some points of the film. The same is true for taking a spectrum by scanning a monochromator with a fixed slit width. 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.
Further Information
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