They showed that electricity and magnetism were two aspects of the same force. The equations also predicted that there should be a form of radiation, which came to be known as electromagnetic radiation. It was soon realized that light was a form of electromagnetic radiation. But the equations predicted that electromagnetic radiation could exist with any wavelength. The various colors of light have wavelengths less than a thousandth of a millimeter.
Much longer wavelengths are possible.
In 1888, Heinrich Hertz built an apparatus that could transmit and receive electromagnetic waves of about 5 meters in length. He used a coil to generate a high voltage spark between two electrodes which served as a transmitter. The detector was a loop of wire with a small gap. A spark at the transmitter produces electromagnetic waves that travel to the detector, producing a spark in the gap. He showed that the waves were polarized, and that they could interfere with each other, just as predicted by theory.