Radio waves are a form of invisible light. They are part of the electromagnetic spectrum, which also includes gamma rays, X-rays, and visible light. Electromagnetic radiation is a combination of electric and magnetic fields that spread through space, carrying energy from one place to another. Any electrical charge that speeds up emits some form of electromagnetic radiation. When a radio wave passes an antenna, it induces a moving electrical charge that can be turned into an audio or other signal to carry information.
The various waves in the electromagnetic spectrum are distinguished by the distances covered by one cycle of the wave (the wavelength), and the number of waves that pass a certain point during a set time period (the frequency). Radio waves can be longer than a football field, or merely the length of a football. The frequencies range from about 3,000 waves per second to 300 billion waves per second.
This broad range is what makes them so useful for practical applications. By tuning a radio receiver to a specific frequency, for example, one can pick up a signal transmitted at that frequency. Radio station call numbers (such as "91.5 FM") show the frequency at which the signal is transmitted.
The Navy was one of the first users of radio waves, for sending Morse code messages between ships and land, but today they permeate nearly every aspect of our daily lives. They are used in telecommunications, including radio and TV broadcasting, wireless phones, pagers, cordless phones, and satellite communications. They are also used in microwave ovens, radar, industrial heaters and sealers, and such medical devices as pacemakers. Astronomers use radio telescopes to "see" distant celestial objects, many of which emit radio waves, using computers to transform the wave data into images that enable them to learn more about the objects' composition, structure, and motion.
Build your own radio!
Here's how you can build your own rudimentary radio with just a fresh 9-volt battery and a coin. Find an AM radio and tune it to a station where you can hear static. Then hold the battery near the antenna and quickly tap the two terminals of the battery with the coin, so that they connect for an instant. You will hear a crackle in the radio, caused by the connection and disconnection of the coin, in essence creating radio waves that the AM radio can receive. You are only transmitting static, and only over a short distance (just a few inches) but you can use the static to tap out Morse code to communicate.