Radio waves have the longest wavelengths in the standard electromagnetic spectrum.
They occupy the low-frequency end of the spectrum, below microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Because all electromagnetic waves travel at the same speed in vacuum, longer wavelength corresponds to lower frequency through the relationship c = fλ.
Radio waves were predicted by James Clerk Maxwell’s electromagnetic theory and experimentally generated and detected by Heinrich Hertz in the late 1880s. Engineers use them for broadcasting, two-way communication, navigation, radar, radio astronomy, and wireless networks.
The boundary between radio waves and microwaves is not universally identical across every classification system. In many modern engineering conventions, microwaves are treated as a higher-frequency part of the radio spectrum. “Longest wavelength” therefore refers to the conventional broad ordering of the electromagnetic spectrum, not a single sharp boundary.