Radio: The Long Waves

Radio waves have the longest wavelengths in the electromagnetic spectrum, stretching from about a millimeter to over 100 kilometers. They carry the least energy per photon and are non-ionizing, so ordinary communication signals do not damage DNA the way X-rays can. At high power, though, radiofrequency energy can still heat biological tissue, which is why transmitters and devices are regulated.

Radio waves penetrate walls, which is why your phone works indoors. They bounce off the ionosphere, letting AM stations reach hundreds of kilometers at night. Astronomers use radio telescopes to detect signals from the edge of the observable universe.