A system of physical chemistry - Index of

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A system of physical chemistry - Index of

THE PHOTO-ELECl'RIC EFFECT 127

GeselL, 13, 967, 19JI ; ibid., 14, 167, 1012). The problem of the

probable structure, constitution, and origin of these light units, as well

as a criticism of Planck's views, has been considered by Sir J. J. Thomson

{Phil. Mag., 1908-10), but it would be quite outside the scope of

this book to attempt anything further than this reference.

To show by an extreme case how/^zf light units it is necessary to

postulate, say, in the visible region of the spectrum under ordinary conditions.

Sir J. J. Thomson has carried out the following calculation.

second would

Light of such intensity that io~* ergs fall on unit area per

be very faint, but would still be visible. If we think of a cylinder of

ether, base i square centimetre and 3 x lo^o cms. in length, then the

above-mentioned intensity is such that 10-* ergs are distributed throughout

this cylinder (at any moment), and the density of the is

light there-

io~*

fore rr, ergs per c.c. Takmg as a mean value for one quantun

3 X 10^"

the number lo"^^ ergs, we see that there will only be one such unit in

every 1000 c.c. of space. The structure of light is therefore of a coarsegrained

character. A further point still remains to be mentioned. The

idea of a point source not radiating uniformly (this being a result of the

heterogeneous nature of emission) seems to be in disagreement with the

ordinary laws of propagation of light equally in all directions. It must

be remembered, however, that any physical source of radiation consists

of a very large number of resonators, so that the total radiating effect

is on the average symmetrical about the source.

The phenomenon of ionisation of a gas or vapour by ultra-violet

light is a further instance of the photo-electric effect. As is well known,

ionisation of a gas may be brought about by means of cathode rays {i.e.

streams of electrons) provided these electrons have attained a certain

velocity, i.e. a kinetic energy ^mvK In an electric discharge this velocity

is reached as a rule before the electrons have travelled far, for they

move with a positive acceleration towards the anode, the act of ionisation

being demonstrated by a glow at some portion of the tube. This

lower limit of kinetic energy which an electron must possess in order to

ionise a gas may be equated to V^-, where V is called the ionisation

potential of the gas, and denotes the smallest voltage which is required

in order to give to an electron the kinetic eneigy ^mv'^ which will just

enable the electron to ionise a neutral gas molecule by impact. Applying

the quantum theory to this case we see that the frequency vq of the

light which will just ionise a gas is given by /zvq = V^ .

The following table contains the most recent values for the ionisation

potential of a number of gases as determined by Hughes and Dixon

{Fhys. Rev., 10, 495 (1917)):—

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