radio broadcasting See broadcasting
radio telescope Combination of radio receiver and antenna, used for observation in radio and radar astronomy. Radio telescopes vary widely, but all have two basic components: a large radio antenna or an antenna array and a radiometer or radio receiver. Because some astronomical radio sources are extremely weak, radio telescopes are usually very large, and only the most sensitive radio receivers are used. The first large fully steer¬ able radio telescope was completed in 1957 at Jodrell Bank, Eng. The world’s largest fully steerable radio telescope is the 360 x 330-ft (110 x 100-m) off-axis antenna operated by the National Radio Astronomy Observatory in Green Bank, W.Va. The largest single radio telescope is the 1,000-ft (305-m) fixed spherical reflector at the Arecibo Observatory in Puerto Rico. The world’s most powerful radio telescope is the Very Large Array in New Mexico, made up of 27 separate mobile parabolic antennas that together provide the angular resolution of a single antenna 22 mi (35 km) in diameter. See photograph above.
Lovell Telescope, a fully steerable radio telescope at Jodrell Bank, Macclesfield, Cheshire, Eng.
JODRELL BANK SCIENCE CENTRE
radio wave Wave from the portion of the electromagnetic spectrum at lower frequencies than microwaves. The wavelengths of radio waves range from thousands of metres to around 30 cm. These correspond to frequen¬ cies as low as 3 Hz and as high as 1 gigahertz (10 9 Hz). Radio-wave com¬ munications signals travel through the air in a straight line, reflect off of clouds or layers of the ionosphere, or are relayed by satellites in space. They are used in standard broadcast radio and television, short-wave radio, navigation and air-traffic control, cellular telephony, and even remote-controlled toys. See illustration on following page.
radioactive series Any of four sets of unstable heavy atomic nuclei that undergo a series of alpha decay and beta decay until a stable nucleus is achieved. The natural series are the thorium series, the uranium series, and the actinium series. These are headed by naturally occurring species of unstable nuclei that have half-lives comparable to the age of the earth. The fourth set, the neptunium series, is headed by neptunium-237, which has a half-life of 2 million years. Its members do not occur naturally but are artificially produced by nuclear reactions and have short half-lives.
radioactivity Property exhibited by certain types of matter of emitting radiation spontaneously. The phenomenon was first reported in 1896 by Henri Becquerel for a uranium salt, and it was soon found that all uranium compounds are radioactive due to the uranium’s radioactivity. In 1898 Marie Curie and her husband discovered two other naturally occurring, strongly radioactive elements, radium and polonium. The radiation is emit¬ ted by unstable atomic nuclei (see nucleus) as they attempt to become more stable. The main processes of radioactivity are alpha decay, beta decay, and gamma decay. In 1934 it was discovered that radioactivity could be induced in ordinary matter by artificial transmutation.
radiocarbon dating See carbon- 14 dating
radiology Branch of medicine that uses radiation for diagnosis (diagnos¬ tic imaging) and treatment (radiation therapy) of disease. Originally, it involved X rays for diagnosis and X rays, gamma rays, and other ionizing radiation for treatment. Diagnostic methods now include isotope scanning (see nuclear medicine), use of nonion- izing radiation, as in ultrasound and magnetic resonance imaging, and radioimmunoassay (in which radio¬ active isotopes in antibodies against hormones detect minute amounts of hormones for diagnosis of endocrine disorders). Radiotherapy now includes, in cancer treatment, radio¬ active hormones and chemothera¬ peutic drugs.
radiotherapy See radiation
THERAPY
radish Annual or biennial plant Radish (Raphanus sativus variety radic-
(Raphanus sativus ) of the mustard u/ a )
family, probably of Oriental origin, ingmar hoimasen
© 2006 Encyclopaedia Britannica, Inc.
1584 I Radishchev ► Raglan
radar
microwave
satellites
cell phones TV
FM radio TV
CB radio
100 GHz
10 GHz
-1 GHz
100 MHz
10 MHz
AM radio 1 MHz
100 kHz
grown for its large, succulent root. Low in calories and high in bulk, radishes have a sharp taste and are usually eaten raw. The shape of the edible portion of the root varies greatly, as does the color (from white through pink to red, purple, and black). Radishes may weigh only a few ounces (U.S. and Euro¬ pean varieties) or, in the case of the Japanese daikon, more than 2 lbs (1 kg).
Radishchev \r3-'dyesh-chif\, Aleksandr (Nikolayevich) (b.
Aug. 31, 1749, Moscow, Russia—<1. Sept. 24, 1802, St. Petersburg) Rus¬ sian political writer. Although from the nobility, he pursued a career as a civil servant, in which he met people from all social classes. Influenced by such writers as Jean-Jacques Rous¬ seau, he wrote A Journey from St. Petersburg to Moscow (1790), in which he described examples of social injustice, hoping that his criti¬ cism of serfdom, autocracy, and cen¬ sorship would enlighten Catherine II. Instead, he was arrested and exiled to Siberia. He was pardoned by Alex¬ ander I in 1801 but committed sui¬ cide a year later. He inspired later revolutionaries, including those who instigated the Decembrist revolt.
radium Chemical element, heaviest alkaline earth metal, chemical symbol Ra, atomic number 88. It was dis¬ covered by Marie Curie and her hus¬ band, Pierre Curie, in 1898 and isolated by 1910. All its isotopes are radioactive (see radioactivity). Radium does not occur free in nature but occurs in natural ores such as pitchblende as a disintegration prod¬ uct of radioactive decay of heavier elements, including uranium. Chemi¬ cally it is highly reactive and has valence 2 in all of its compounds. Its use in medicine (see radiation therapy; radiology; nuclear medicine) has declined because of its cost, and its use in consumer goods (to illumi¬ nate watch and clock hands and numbers, as well as instrument dials) was halted because it can cause radiation injury. It is still used for some radiography and as a source of NEUTRONS.
radon \'ra-,dan\ Chemical element, chemical symbol Rn, atomic number 86. The heaviest noble gas, it is colourless, odourless, tasteless, radioactive (see radioactivity), and almost completely unreactive (form¬ ing compounds only with fluorine). It is rare in nature because all its iso¬ topes are short-lived and because radium, its source, is scarce. Radon seeps from certain soils and rocks (such as granite) into the atmosphere and can accumulate in poorly venti¬ lated spaces near ground level, including house basements; in some regions of the world the use of such spaces is believed to increase the risk
10 kHz
Radio waves lie at the low-frequency end of the electromagnetic spectrum. They are primarily used in various types of communications signals. Also of importance is the detection of natural radio sources in radio and radar astronomy. A few applications are shown at their approximate positions in the spectrum (on a logarithmic scale). Microwaves are a subset of the radio spectrum, ranging from about 1 to 1000 mm in wavelength, or a fre¬ quency between about 1 and 100 GHz. The microwave region is used especially in various forms of radar, in communications with spacecraft and satellites (as in the Global Positioning System), and in microwave ovens. Amateur communications, such as CB (citizens' band) and short-wave radio, occur around 10 MHz. Marine naviga¬ tion and communications systems oper¬ ate especially below 1 MHz. Other devices or systems using radio waves include metal detectors, loran, and magnetic resonance imaging.