A system of physical chemistry - Index of

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A system of physical chemistry - Index of

124

A SYSTEM OF PHYSICAL CHEMISTRY

has been found that the photo-electric

effect increases proportionally

to this intensity, because the number of electrons emitted increases in

the same proportion. As long, however, as the frequency of the light

is maintained constant, the velocity of the electrons is also constant.

On the other hand, working with constant intensity of light but varying

the quality or it frequency has been found that, on increasing the

frequency of the light, the photo-electric effect increases, because the

to Lenard and

speed of the electrons is now increased. According

Ladenburg, when the wave-length is thus shortened (at constant in-

tensity), the velocity of the electrons emitted increases proportionally

to the It is

frequency. now known that the square of the is speed

proportional to the frequency of the incident light. Below a certain

wave-length or frequency, known as the threshold wave-length, the

normal photo-electric effect is not observable. That is, with wavelengths

longer than a certain value Aq— characteristic of the substance

under examination— no electrons are emitted. Hughes has determined

the value of Xq for a number of substances, using fresh surfaces, obtained

by distillation of the substance in vacuo. These values are given in

the following table :—

Normal Photo-electric Effect. Threshold Wave-lengths (Xq).

Substance.

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