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]]. | 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]]. |