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radiation (rāˌdēāˈshən), term applied to the emission and transmission of energy through space or through a material medium and also to the radiated energy itself. In its widest sense the term includes electromagnetic, acoustic, and particle radiation, and all forms of ionizing radiation. Commonly radiation refers to the electromagnetic spectrum, which, in order of decreasing wavelength, includes radio, microwave, infrared, visible-light, ultraviolet, X-ray, and gamma-ray emissions. All of these travel through space at the speed of light (c.300,000 km/186,000 mi per sec) but differ in wavelength and frequency. According to the quantum theory, the energy carried in the form of electromagnetic radiation may be viewed as made up of tiny bundles or packets, each bundle being known as a photon. The sun is the source of much radiant energy in the form of sunlight and heat. Heat radiation is infrared radiation. All types of electromagnetic radiation can be reflected and absorbed in the same manner as is visible light. Acoustic radiation, propagated as sound waves, may be sonic (in the frequency range from 16 to 20,000 cycles per sec), infrasonic, or subsonic (frequency less than 16 cycles per sec), and ultrasonic (frequency greater than 20,000 cycles per sec). Examples of particle radiation are alpha and beta rays in radioactivity, and many kinds of atomic and subatomic particles such as electrons, mesons, neutrons, protons, and heavier nuclei (see cosmic rays). Radiation is usually considered to travel from a source in straight lines, but its path may be affected by external factors; for instance, charged particles travel in curved paths in magnetic fields. The Van Allen radiation belts consist of charged particles trapped in the earth's magnetic field.
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The emission and propagation of energy; also, the emitted energy itself. The etymology of the word implies that the energy propagates rectilinearly, and in a limited sense, this holds for the many different types of radiation encountered.

The major types of radiation may be described as electromagnetic, acoustic, and particle, and within these major divisions there are many subdivisions. Electromagnetic radiation is classified roughly in order of decreasing wavelength as radio, microwave, visible, ultraviolet, x-rays, and γ-rays. Acoustic or sound radiation may be classified by frequency as infrasonic, sonic, or ultrasonic in order of increasing frequency, with sonic being between about 16 and 20,000 Hz. The traditional examples of particle radiation are the α‒ and β-rays of radioactivity. See Electromagnetic radiation, Radioactivity, Sound

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


(ray-dee-ay -shŏn) See electromagnetic radiation; energy transport.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006


A method of surveying in which points are located by knowledge of their distances and directions from a central point.
The emission and propagation of waves transmitting energy through space or through some medium; for example, the emission and propagation of electromagnetic, sound, or elastic waves.
The energy transmitted by waves through space or some medium; when unqualified, usually refers to electromagnetic radiation. Also known as radiant energy.
A stream of particles, such as electrons, neutrons, protons, α-particles, or high-energy photons, or a mixture of these.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


The transmission of heat through space by means of electromagnetic waves; the heat energy passes through the air between the source and the heated body without heating the intervening air appreciably.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.


i. The process of heat transfer in wave form without the use or necessity of a transmitting medium. The insolation, or radiant energy, received by the earth from the sun is an example of radiation.
ii. The transfer of energy in the form of electromagnetic waves through either a vacuum or air.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved


1. Physics
a. the emission or transfer of radiant energy as particles, electromagnetic waves, sound, etc.
b. the particles, etc., emitted, esp the particles and gamma rays emitted in nuclear decay
2. Med treatment using a radioactive substance
3. Anatomy a group of nerve fibres that diverge from their common source
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

electromagnetic radiation

The energy that radiates from all things in nature and from man-made electrical and electronic systems. Electromagnetic radiation includes cosmic rays, gamma rays, x-rays, ultraviolet light, visible light, infrared light, radar, microwaves, TV, radio, cellphones and all electronic transmission systems. Electromagnetic radiation is made up of an electromagnetic field (EMF), which comprises an electric field and a magnetic field that move at right angles to each other at the speed of light. See spectrum, microwave and electromagnetic hypersensitivity.

radiation hardened

Refers to electronic products used in space, satellite, nuclear plant and military applications. Radiation hardened devices are built to withstand cosmic rays and other natural electromagnetic radiation, as well as nuclear explosions. The effects of such radiation can be a temporary alteration or a slow degradation of the semiconductor elements in memory cells and transistors. If either a cell or transistor is induced to change its state, it can cause a program to crash (see abend). As the elements in a chip are made smaller, radiation has an increasingly greater and harmful influence.

"Rad hard" products are highly insulated from the outside world. In addition, devices may be built with redundant components at the system level or at the circuit level, and error-correcting memories can detect and correct a memory failure. See electromagnetic radiation.

A Wide Resistance Difference
This C-RAM phase change 4MB memory chip is shown without the cover and before the leads are cut. When the bit is 0, the memory cell resistance is 5,000 ohms, but 100,000 ohms when a 1. Due to the huge resistance difference between a 0 and 1, the chip is inherently radiation hardened (see phase change memory). (Image courtesy of BAE Systems, www.baesystems.com)
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