Radio waves
The longest-wavelength electromagnetic waves, travelling at the speed of light and used to carry sound and signals without wires.
Radio waves are electromagnetic waves, the same basic thing as light, but with much longer wavelengths: more than about a millimetre, at frequencies below roughly 300 gigahertz. They are produced whenever electric charges oscillate, as in a transmitting aerial, and in turn they make charges oscillate in a receiving aerial. In empty space they travel at the speed of light, 299,792,458 metres per second.
James Clerk Maxwell's theory of electromagnetism, published in the 1860s, predicted such waves. Heinrich Hertz made and detected them in his laboratory in 1887, and around 1894 to 1895 Guglielmo Marconi built the first practical transmitters and receivers for wireless telegraphy, sending Morse code through the air.
Early wireless had an obvious weakness: anyone with a receiver tuned to roughly the same frequency could listen in, and anyone with a transmitter could butt in. Better tuning, which uses resonance to pick out a narrow band of frequencies, made signals easier to separate, but radio has never been private by nature.
Some radio frequencies travel in useful ways. Medium-wave and shortwave signals can bounce off charged layers high in the atmosphere, the ionosphere, and come back down far beyond the horizon, especially at night, which is how a broadcast can be heard across an ocean.
A common misconception is that radio waves are a kind of sound. They are not; a radio set turns them into sound with a loudspeaker.
See it in action
Premieres 20 NovA Moon of Jupiter Clocked the Speed of Light1676 · Paris Observatory
Coming soonFixing a 1977 Computer One Light-Day AwayNov 2023 – Apr 2024 · Voyager 1, interstellar space
Premieres 22 NovThe Magician Who Trolled MarconiJune 1903 · Royal Institution, London (and Poldhu / Porthcurno, Cornwall)Coming up in future stories
- He Rang a Bell Through Two Walls in 1895 1895 · Presidency College, Calcutta
- South Africa's Telescope Found Bubbles Around Our Black Hole 11 September 2019 · MeerKAT, Northern Cape, South Africa
- The Soviet Children's Gift That Listened for Seven Years 1945–1952 · Spaso House, Moscow
- The 'Wow!' Signal May Have Been a Cosmic Laser 15 Aug 1977 · Big Ear radio telescope, Ohio