Today's video from the Cabinet of Physics illustrates the malleable nature of light.
A bright white light illuminates a screen. The curator's gloved hand inserts a prism into the white beam. On the screen, the former white spot becomes a wide bar of many colors—a spectrum.
An array of seven small mirrors intercepts the spectrum. Each mirror reflects an intensely-colored spot onto another screen. The mirrors may be adjusted to make their spots overlap, producing new colors. Finally, when all seven different colors overlap, a new spot of white is created.
This simple apparatus taught students how different wavelengths of light could be extracted or recombined to manipulate colors. Computer users observe the same thing every day. If there's anything white on the screen you're looking at, it may well be a combination of light from an array of tiny single-color light sources, such as light-emitting diodes or the phosphor dots of a cathode-ray tube. (Or not, depending on the technology used in your display.)
So the recombination of light taught by the Seven Mirrors is a ubiquitous phenomenon in today's screen-filled world.
The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments. The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.