A system of physical chemistry - Index of

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A system of physical chemistry - Index of

THE PHOTO-ELECTRIC EFFECT 125

Camb. Phil. Soc, 14, 421, 1908): The [radiant] energy travelling

outwards [from the radiating source] with the wave is not spread

uniformly over the wave front, but is concentrated on those parts of

the front where the pulses are travelling along the lines of force ;'^

these parts correspond to the bright specks, the rest

of the wave is thus collected

to the dark

into isolated

ground. . . . The energy

regions, these regions being the portions of the lines of force occupied

by the pulses or wave motion. In fact, from this point of view, the

distribution of energy is very like that contemplated on the old emission

theory, according to which the energy was located on moving particles,

sparsely disseminated throughout space. The energy is, as it were,

done up into bundles, and the energy in any particular bundle does

not change as the bundle travels along the line of force. Thus, when

light falls upon a metal plate, if we increase the distance of the source

of light, we shall diminish the number of these different bundles or units

falling on a given area of the metal, but we shall not diminish the

energy in the individual units ; thus any effect which can be produced

by a unit by itself, will, when the source of light is removed to a greater

distance, take place less frequently it is true, but when it does take

place it will be of the same character as when the intensity of the light

was stronger. This is, I think, the explanation of the remarkable

result discovered by Lenard, that though the number of corpuscles

emitted by a piece of metal exposed to ultra-violet light increases [as

intensity increases], the velocity with which individual corpuscles come

from the metal does not depend upon the intensity of the light.

this result stood alone, we might suppose that it indicated that

If

the

forces which expel the corpuscles from the metal are not the electron

forces in the light wave incident on the metal, but that the corpuscles

are ejected by the explosion of some of the molecules of the metal

which have been put into an unstable state by the incidence of the

light ; if this were the case the velocity of the corpuscle would be

determined by the properties of the atom of the metal, and not by

the intensity of the light, which merely acts as a the

trigger

explosion. Some experiments made quite recently by

to start

Dr. E.

Ladenburg make, however, this last explanation exceedingly improbable.

Ladenburg has investigated the velocities of corpuscles emitted under

the action of ultra-violet light of different wave-lengths, and finds that

the velocity varies continuously with the frequency ; according to his

interpretations of his experiments, the velocity is directly proportional

to the frequency.^ Thus, though the velocity of the corpuscles is

independent of the intensity of the light, it varies in apparently quite a

continuous way with the quality of the light this would be ;

very improbable

if the corpuscles were expelled by an explosion of the molecule.

1 Thomson assumes that the ether has disseminated through it discrete lines of

electric force, these being in a state of tension. The light consists of transverse

vibrations travelling along these lines.

^ As already stated, the evidence now available goes to show that the square of

the velocity of the electron is proportional to the frequency of the light.

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