Radio wave
Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths greater than 1 millimeter (3⁄64 inch), about the diameter of a grain of rice.
Radio waves with frequencies above about 1 GHz and wavelengths shorter than 30 centimeters are called microwaves.
Radio waves are generated by charged particles undergoing acceleration, such as time-varying electric currents.
Radio waves were first predicted by the theory of electromagnetism that was proposed in 1867 by Scottish mathematical physicist James Clerk Maxwell.
[5] His mathematical theory, now called Maxwell's equations, predicted that a coupled electric and magnetic field could travel through space as an "electromagnetic wave".
Radio waves are produced artificially by time-varying electric currents, consisting of electrons flowing back and forth in a specially shaped metal conductor called an antenna.
From quantum mechanics, like other electromagnetic radiation such as light, radio waves can alternatively be regarded as streams of uncharged elementary particles called photons.
Air is tenuous enough that in the Earth's atmosphere radio waves travel at very nearly the speed of light.
The relation of frequency and wavelength in a radio wave traveling in vacuum or air is where Equivalently,
A plane-polarized radio wave has an electric field that oscillates in a plane perpendicular to the direction of motion.
In a circularly polarized wave the electric field at any point rotates about the direction of travel, once per cycle.
The polarization of radio waves is determined by a quantum mechanical property of the photons called their spin.
Right circularly polarized radio waves consist of photons spinning in a right hand sense.
Plane polarized radio waves consist of photons in a quantum superposition of right and left hand spin states.
Radio waves passing through different environments experience reflection, refraction, polarization, diffraction, and absorption.
Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others.
Above 300 GHz, in the terahertz band, virtually all the power is absorbed within a few meters, so the atmosphere is effectively opaque.
At the sending end, the information to be sent, in the form of a time-varying electrical signal, is applied to a radio transmitter.
At the receiver, the oscillating electric and magnetic fields of the incoming radio wave push the electrons in the receiving antenna back and forth, creating a tiny oscillating voltage which is a weaker replica of the current in the transmitting antenna.
The oscillating electric field of the wave causes polar molecules to vibrate back and forth, increasing the temperature; this is how a microwave oven cooks food.
A strong enough beam of radio waves can penetrate the eye and heat the lens enough to cause cataracts.
Radiofrequency electromagnetic fields have been classified by the International Agency for Research on Cancer (IARC) as having "limited evidence" for its effects on humans and animals.
[22][23] There is weak mechanistic evidence of cancer risk via personal exposure to RF-EMF from mobile telephones.
Power density is most accurately used when the point of measurement is far enough away from the RF emitter to be located in what is referred to as the far field zone of the radiation pattern.
When speaking of frequencies in the microwave range and higher, power density is usually used to express intensity since exposures that might occur would likely be in the far field zone.
