Editing Radio wave (section)

== Generation and reception ==
[[File:Dipole receiving antenna animation 6 300ms.gif|thumb |upright=1.5 |Animated diagram of a [[half-wave dipole]] antenna receiving a radio wave. The antenna consists of two metal rods connected to a receiver {{mvar|R}}. The [[electric field]] (<span style="color:green;">{{mvar|E}}, green arrows</span>) of the incoming wave results in oscillation of the [[electron]]s in the rods, charging the ends alternately positive <span style="color:red;">(+)</span> and negative <span style="color:blue;">(−)</span>. Since the length of the antenna is one half the [[wavelength]] of the wave, the oscillating field induces [[standing wave]]s of voltage (<span style="color:red;">{{mvar|V}}, represented by red band</span>) and current in the rods. The oscillating currents (black arrows) flow down the transmission line and through the receiver (represented by the resistance {{mvar|R}}).]]

Radio waves are radiated by [[charged particle]]s when they are [[acceleration|accelerated]].  Natural sources of radio waves include [[radio noise]] produced by [[lightning]] and other natural processes in the Earth's atmosphere, and [[astronomical radio source]]s in space such as the Sun, galaxies and nebulas.  All warm objects radiate high frequency radio waves ([[microwave]]s) as part of their [[black body radiation]].

Radio waves are produced artificially by time-varying [[electric current]]s, consisting of [[electron]]s flowing back and forth in a specially shaped metal conductor called an [[antenna (radio)|antenna]]. An electronic device called a [[radio transmitter]] applies oscillating electric current to the antenna, and the antenna radiates the power as radio waves. Radio waves are received by another antenna attached to a [[radio receiver]]. When radio waves strike the receiving antenna they push the electrons in the metal back and forth, creating tiny oscillating currents which are detected by the receiver.

From [[quantum mechanics]], like other electromagnetic radiation such as light, radio waves can alternatively be regarded as streams of uncharged [[elementary particle]]s called ''[[photon]]s''.<ref name="Gosling-1998">{{cite book |last=Gosling |first=William |url=http://nvhrbiblio.nl/biblio/boek/Gosling%20-%20Radio%20antennas%20and%20propagation.pdf |title=Radio Antennas and Propagation |date=1998 |publisher=Newnes |isbn=0750637412 |pages=2, 12 |access-date=2021-10-28 |archive-url=https://web.archive.org/web/20211028223233/http://nvhrbiblio.nl/biblio/boek/Gosling%20-%20Radio%20antennas%20and%20propagation.pdf |archive-date=2021-10-28 |url-status=live}}</ref>  In an antenna transmitting radio waves, the electrons in the antenna emit the energy in discrete packets called radio photons, while in a receiving antenna the electrons absorb the energy as radio photons.  An antenna is a [[coherence (physics)|coherent]] emitter of photons, like a [[laser]], so the radio photons are all [[in phase]].<ref name="Shore">{{cite book |last=Shore |first=Bruce W. |url=https://books.google.com/books?id=4Gr8DwAAQBAJ&dq=coherent&pg=PA54 |title=Our Changing Views of Photons: A Tutorial Memoir |date=2020 |publisher=Oxford University Press |isbn=9780192607645 |pages=54 |access-date=2021-12-04 |archive-url=https://web.archive.org/web/20240922182240/https://books.google.com/books?id=4Gr8DwAAQBAJ&dq=coherent&pg=PA54#v=onepage&q=coherent&f=false |archive-date=2024-09-22 |url-status=live}}</ref><ref name="Gosling-1998" /> However, from [[Planck's relation]] <math>E = h\nu</math>, the energy of individual radio photons is extremely small,<ref name="Gosling-1998" /> from 10<sup>−22</sup> to 10<sup>−30</sup> [[joule]]s. So the antenna of even a very low power transmitter emits an enormous number of photons every second.  Therefore, except for certain [[molecular electron transition]] processes such as atoms in a [[maser]] emitting microwave photons, radio wave emission and absorption is usually regarded as a continuous [[classical electromagnetism|classical]] process, governed by [[Maxwell's equations]].