Microwave is a short radio wave. It varies from 1 to 300 millimeters (about 1/25 to 12 inches) in length. Like light waves, microwaves may be reflected and concentrated. But they pass easily through rain, smoke, and fog, which block light waves. They can also pass through the ionosphere, which surrounds the earth and blocks or reflects longer radio waves. Thus, microwaves are well suited for long-distance, satellite, and space communications and for control of navigation.