Newton first uses the word spectrum (Latin for "appearance" or "apparition") in print in 1671 in describing his experiments in optics.

Newton has observed that when a narrow beam of sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different colored bands.

Newton hypothesizes that light is made up of "corpuscles" (particles) of different colors, and that the different colors of light move at different speeds in transparent matter, with red light moving more quickly in glass than violet.

The result is that red light bends (refracted) less sharply than violet as it passes through the prism, creating a spectrum of colors.

Newton divides the spectrum into seven named colors: red, orange, yellow, green, blue, indigo, and violet. (Some schoolchildren memorize this order using the mnemonic ROY G. BIV.)

He has chosen seven colors out of a belief, derived from the ancient Greek sophists, that there is a connection between the colors, the musical notes, the known objects in the solar system, and the days of the week.