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radiation, flow of atomic and subatomic particles and of waves, such as

those that characterize heat rays, light rays, and X rays. All matter is constantlybombarded with radiation of both types from cosmic and terrestrial sources. This article

delineates the properties and behaviour of radiation and the matter with which it

interacts and describes how energy is transferred from radiation to its surroundings.

Considerable attention is devoted to the consequences of such an energy transfer to

living matter, including the normal effects on many life processes (e.g., photosynthesis

in plants and vision in animals) and the abnormal or injurious effects that result from the

exposure of organisms to unusual types of radiation or to increased amounts of the

radiations commonly encountered in nature. The applications of various forms of 

radiation in medicine and technological fields are touched upon as well.

General background Table Of Contents

Types of radiationRadiation may be thought of as energy in motion either at speeds equal to the speed

of  light in free space — approximately 3 × 1010 centimetres (186,000 miles) per second — 

or at speeds less than that of light but appreciably greater than thermal velocities (e.g.,

the velocities of molecules forming a sample of air). The first type constitutes the

spectrum of  electromagnetic radiation that includes radio waves, microwaves, infrared

rays, visible light, ultraviolet rays, X rays, and gamma rays, as well as the neutrino (see

below). These are all characterized by zero mass when (theoretically) at rest. Thesecond type includes such particles as electrons, protons, and neutrons. In a state of rest,

these particles have mass and are the constituents of atoms and atomic nuclei. When

such forms of particulate matter travel at high velocities, they are regarded as radiation.

In short, the two broad classes of radiation are unambiguously differentiated by their

speed of propagation and corresponding presence or absence of  rest mass. In the

discussion that follows, those of the first category are referred to as ―electromagnetic

rays‖ (plus the neutrino) and those of the second as ―matter rays.‖ 

At one time, electromagnetic rays were thought to be inherently wavelike in character — 

namely, that they spread out in space and are able to exhibit interference when they

come together from two or more sources. (Such behaviour is typified by water waves inthe way they propagate and periodically reinforce and cancel one another.) Matter rays,

on the other hand, were considered to be inherently particle-like in character — i.e.,

localized in space and incapable of interference. During the early 1900s, however,

major experiments and attendant theories revealed that all forms of radiation, under

appropriate conditions, can exhibit both particle-like and wavelike behaviour. This is

referred to as the wave – particle duality and provides in large part the foundation for the

modern quantum theory of matter and radiation. The wave behaviour of radiation is

apparent in its propagation through space, while the particle behaviour is revealed by

the nature of interactions with matter. Because of this, care must be exercised to use the

terms waves and particles only when appropriate.

ELECTROMAGNETIC RAYS AND NEUTRINOS

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VISIBLE LIGHT AND THE OTHER COMPONENTS OF THE 

 ELECTROMAGNETIC SPECTRUM According to the theory of relativity, the velocity of light is a fixed quantity independent

of the velocity of the emitter, the absorber, or a presumably independent observer, all

three of which do affect the velocities of common wavelike disturbances such as sound.In an extended definition, the term light embraces the totality of electromagnetic

radiation. It includes the following: the long electromagnetic waves predicted by the

Scottish physicist James Clerk Maxwell in 1864 and discovered by the German

physicist Heinrich Hertz in 1887 (now called radio waves); infrared and ultraviolet rays;

the X rays discovered in 1895 by Wilhelm Conrad Röntgen of Germany; the gamma

rays that accompany many radioactive-decay processes; and some even more energetic

(with higher energy) X rays and gamma rays produced as the normal accompaniment of 

the operations of ultrahigh-energy machines (i.e., particle accelerators such as the Van

de Graaff generator, the cyclotron and its variants, and the linear accelerator).

The behaviour of light seems to have interested ancient philosophers but without

stimulating them to experiment, though all of them were impressed by vision. The first

meaningful optical experiments on light were performed by the English physicist and

mathematician Isaac Newton (beginning in 1666), who showed (1) that white

light diffracted by a prism into its various colours can be reconstituted into white light

by a prism oppositely arranged and (2) that light of a particular colour selected from the

diffracted spectrum of a prism cannot be further diffracted into beams of other colour by

an additional prism. Newton hypothesized that light is corpuscular in its nature, each

colour represented by a different particle speed, an erroneous assumption. Furthermore,

in order to account for the refraction of light, the corpuscular theory required, contrary

to the wave theory of the Dutch scientist Christiaan Huygens (developed at about the

same time), that light corpuscles travel with greater velocity in the denser medium.Support for the wave theory came in the electromagnetic theory of Maxwell (1864) and

the subsequent discoveries of Hertz and of Röntgen of both the very long and the very

short waves Maxwell had included in his theory. The German physicist Max

Planck  proposed a quantum theory of radiation to counter some of the difficulties

associated with the wave theory of light, and in 1905 Einstein proposed that light is

composed of quanta (later called photons). Thus, experiment and theory had led around

from particles (of Newton) that behave like waves (Huygens) to waves (Maxwell) that

behave like particles (Einstein), the apparent velocity of which is unaffected by the

velocity of the source or the velocity of the receiver. Furthermore it was found, in 1922,

that the shorter-wavelength electromagnetic radiations(e.g., X rays) have momentum

such as may be expected of particles, part of which can be transferred to electrons withwhich they collide (i.e., the Compton effect).

 NEUTRINOS  AND  ANTINEUTRINOS Neutrinos and their antiparticles are forms of radiation similar to electromagnetic rays in

that they travel at the speed of light and have little or no rest mass and zero charge.

They too are produced by ultrahigh-energy particle accelerators and certain types

of  radioactive decay. 

o far, nothing has been written about how an amplifying medium amplifies light. This will bedealt with here and in the next section. We must begin with an account of how light can interact

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with individual atoms within an amplifying medium ("atoms" will be used to include moleculesand ions). Atoms consist of a positively charged core (nucleus) which is surrounded bynegatively charged electrons. According to the quantum mechanical description of an atom, theenergy of an atomic electron can have only certain values and these are represented by energylevels. The electrons can be thought of as orbiting the nucleus, those with the largest energyorbiting at greater distances from the nuclear core. There are many energy levels that an

electron within an atom can occupy, but here we will consider only two. Also, we will consideronly the electrons in the outer orbits of the atom as these can most easily be raised to higherunfilled energy states. 

Absorption and Spontaneous Emission 

The processes of the absorption and spontaneous emission of light are illustrated below: 

A photon of light is absorbed by an atom in which one of the outer electrons is initially in a lowenergy state denoted by 0. The energy of the atom is raised to the upper energy level, 1, andremains in this excited state for a period of time that is typically less than 10

-6second. It then

spontaneously returns to the lower state, 0, with the emission of a photon of light. Absorption isreferred to as a resonant process because the energy of the absorbed photon must be equal tothe difference in energy between the levels 0 and 1. This means that only photons of aparticular frequency (or wavelength) will be absorbed. Similarly, the photon emitted will haveenergy equal to the difference in energy between the two energy levels. These commonprocesses of absorption and spontaneous emission cannot give rise to the amplification of light.The best that can be achieved is that for every photon absorbed, another is emitted. 

Stimulated Emission 

Stimulated emission is a very uncommon process in nature but it is central to the operation oflasers. 

Above it was stated that an atom in a high energy, or excited, state can return to the lower statespontaneously. However, if a photon of light interacts with the excited atom, it can stimulate areturn to the lower state. One photon interacting with an excited atom results in two photons

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being emitted. Furthermore, the two emitted photons are said to be in phase, i.e. thinking ofthem as waves, the crest of the wave associated with one photon occurs at the same time as onthe wave associated with the other. This feature ensures that there is a fixed phase relationshipbetween light radiated from different atoms in the amplifying medium and results in the laserbeam produced having the property of coherence. Stimulated emission is the process that cangive rise to the amplification of light. As with absorption, it is a resonant process; the energy of

the incoming photon of light must match the difference in energy between the two energy levels.Furthermore, if we consider a photon of light interacting with a single atom, stimulated emissionis just as likely as absorption; which process occurs depends upon whether the atom is initiallyin the lower or the upper energy level. However, under most conditions, stimulated emissiondoes not occur to a significant extent. The reason is that, under most conditions, that is, underconditions of thermal equilibrium, there will be far more atoms in the lower energy level, 0, thanin the upper level, 1, so that absorption will be much more common than stimulated emission. Ifstimulated emission is to predominate, we must have more atoms in the higher energy statethan in the lower one. This unusual condition is referred to as a population inversion and it isnecessary to create a population inversion for laser action to occur.