bei48482_FM

144 Chapter Four

When white light, which contains all wavelengths, is passed through hydrogen gas,

photons of those wavelengths that correspond to transitions between energy levels are

absorbed. The resulting excited hydrogen atoms reradiate their excitation energy almost

at once, but these photons come off in random directions with only a few in the same

direction as the original beam of white light (Fig. 4.20). The dark lines in an absorp-

tion spectrum are therefore never completely black but only appear so by contrast with

the bright background. We expect the lines in the absorption spectrum of any element

to coincide with those in its emission spectrum that represent transitions to the ground

state, which agrees with observation (see Fig. 4.9).

Franck-Hertz Experiment

Atomic spectra are not the only way to investigate energy levels inside atoms. A series

of experiments based on excitation by collision was performed by James Franck and

Gustav Hertz (a nephew of Heinrich Hertz) starting in 1914. These experiments demon-

strated that atomic energy levels indeed exist and, furthermore, that the ones found in

this way are the same as those suggested by line spectra.

Franck and Hertz bombarded the vapors of various elements with electrons of known

energy, using an apparatus like that shown in Fig. 4.21. A small potential difference

V 0 between the grid and collecting plate prevents electrons having energies less than

a certain minimum from contributing to the current Ithrough the ammeter. As the

accelerating potential Vis increased, more and more electrons arrive at the plate and

I rises (Fig. 4.22).

Figure 4.20The dark lines in an absorption spectrum are never totally dark.

White light

Absorbed wavelength

Transmitted

wavelengths

The absorbed light

is reradiated in all

directions

Gas

Gas

Figure 4.21Apparatus for the Franck-Hertz experiment.

Filament Grid Plate

VV 0

A

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