14.2 - Radio Waves
Radio waves carry communication signals
- Radio waves are used mainly for communication - they carry television and radio broadcasts from one place to another.
- Radio waves have wavelengths longer than about 10 cm, and how a radio wave travels from transmitter to receiver depends on its wavelength.
- Longer-wavelength radio waves can be received over very large distances.
- The very short wavelengths used for TV and FM radio only reach a receiver in line of sight of the transmitter, because they don't bend or travel far through obstacles.
How radio wavelengths travel to a receiver
- Long-wave radio (wavelengths of about 1-10 km) - Can be received from halfway round the world because the long wavelengths diffract (bend) around the curved surface of the Earth, hills, and into tunnels.
- Short-wave radio (wavelengths of about 10-100 m) - Can be received at long distances because it is reflected by the ionosphere (an electrically charged layer high in the Earth's atmosphere).
- TV and FM radio - Use very short wavelengths that do not diffract around obstacles or travel far through buildings, so the receiver must be in line of sight of the transmitter.
Bluetooth uses radio waves
- Bluetooth uses radio waves to send data wirelessly over short distances between devices (for example, a phone and wireless headphones).
- The wavelengths used are even shorter than those for TV and FM radio.
- The waves can pass through walls but are weakened as they do so, which keeps the range short and reduces interference between nearby devices.
Radio telescopes detect radio waves from space
- Many objects in space, including distant galaxies, emit radio waves that radio telescopes on Earth can detect.
- These radio waves give astronomers information about the composition and motion of objects in outer space.
RFID tags use radio waves
- RFID (radio frequency identification) transfers data using radio waves.
- An RFID tag lets an object be identified at short range without contact (for example, library books, livestock, and car keyfobs).
TermDefinition
IonosphereAn electrically charged layer high in the Earth's atmosphere that reflects short-wave radio signals.