All the colours of the spectrum
I have a prism hanging in my window. At this time of the year the spectrum of light is thrown across the room in the afternoon.
Newton 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 hypothesized that light was made up of "corpuscles" (particles) of different colors, and that the different colors of light moved 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 divided the spectrum into seven named colors: red, orange, yellow, green, blue, indigo, and violet. (Some schoolchildren memorize this order using the mnemonics ROY G. BIV or Richard Of York Gave Battle In Vain.) He chose seven colors out of a belief, derived from the ancient Greek sophists, that there was a connection between the colors, the musical notes, the known objects in the solar system, and the days of the week. The human eye is relatively insensitive to indigo's frequencies, and some otherwise well-sighted people cannot distinguish indigo from blue and violet.