interruptions

In traveling to us and through us, waves and rays may bring not just energy but also communicate meaning. Waves that are constant, for example as in the beam of a flashlight, cannot convey any information. But if that beam is interrupted, or if its brightness can be made to change, then it can carry a message. This is how all waveborne communications work. Patterns of energy arrive from energy sources that are high or low, loud or soft, light or dark, one color or another. In this way, sound and light rays bring us music, voices, words on a page and expressions on faces. By converting one kind of wave into another, and also by storing their energy, waves can be made to carry sounds and images around the world and far beyond it -- and to transport them through time.

-- David Macaulay The Way Things Work