Forms of EM Waves:

The invention of EM fields led eventually to the diversity of wireless communications systems we know nowadays. The Radio waves are not the only type of EM radiation. Since the frequency rises above that of conventional radio, we encounter new types. At first comes the microwaves. Then comes infrared (IR) or "heat rays."Afterward that comes visible light, x-rays, ultraviolet (UV) radiation, & gamma (γ) rays.

In the reverse, and less generally imagined, logic, EM fields can exist at frequencies far below those of radio signals. In hypothesis, an EM wave can go through one complete cycle each and every hour, day, year, thousand years, or million years. Some of the astronomers suspect that stars and galaxies produce EM fields with period of years, centuries, & millennia.