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# Light waves and particles

Scientists have attempted to answer the question "what is light?" for hundreds of years. The English scientist Isaac Newton (1642–1727) believed light was made up of microscopic balls or particles. The Dutch scientist Christiaan Huygens (1629–95) proposed that light travelled as waves, rather like sound or ripples on the surface of water. But when it came to explaining how light is absorbed and emitted by atoms, light has to be described not as waves but as packets of energy, or particles called photons.

## Waves

A light wave travels in one direction, wobbling up and down as it does so. It has crests (or "peaks") and troughs, a bit like ripples on a pond. As it moves, there is always the same distance between crests and troughs; this distance is called the wavelength. The height of a crest or the depth of a trough is known as the wave's amplitude. The greater the amplitude, the greater the wave's energy. If you could count the number of wave crests that went by in one second, that number would be the frequency of the wave: this is always the same for that wave. The unit of frequency, a measure used to compare frequencies of all waves, including sound, radio and light waves, is the hertz (Hz), named after the German physicist Heinrich Hertz (1857–94).