“spectrum: Arrangement according to wavelength (or frequency) of electromagnetic radiation. The visible, ‘rainbow’ spectrum is a portion of the electromagnetic spectrum that is visible as light to the human eye. Some sources emit only certain wavelengths and produce and emission spectrum of bright lines with dark spaces between. Such line spectra are characteristic of the elements that emit the radiation. A band spectrum consists of groups of wavelengths so close together that the lines appear to form a continuous band. Atoms and molecules absorb certain wavelengths and so remove them from a complete spectrum; the resulting absorption spectrum contains dark lines or bands at these wavelengths.”
Excerpted/Adapted from: Stevens, Mark A., Ed. Merriam-Webster’s Collegiate Encyclopedia. Springfield, Massachusetts: Merriam-Webster, 2000.