Spectrum - (n.)

A plot of wavelengths (or equivalent measures of cyclicity) against amplitude (or other measures of energy content). For visible light, the spectrum includes all colors between violet and dark red. For electromagnetic radiation, the spectrum includes everything from gamma radiation and x-rays to radio waves, with visible light in between. For water waves, the spectrum includes everything between wind ripples to tides. For seismic waves, the spectrum ranges from barely noticeable vibrations to the great long-wave oscillations that are so dangerous to buildings. For climate fluctuations, the spectrum spans the gamut from daily temperature fluctuations to annual seasons to the great ice age cycles. Everything we know about distant stars and galaxies comes from analyzing the spectrum of their electromagnetic radiation, and almost everything we know about the interior of the Earth comes from analyzing the spectrum of various types of seismic waves. The mathematics of spectral analysis are based on the work of the great French physicist and mathematician J.B. Fourier (1768-1830).