Franck-Hertz experiment

Introduction:

In Frank-Hertz experiment the critical potential or existence of discrete energy levels of Hg-atom is proved by the method of excitation by collision. In this experiment there is collision between electron and Hg-atom.

The experiment setup consists of a filament F connected to low tension battery. For the source of therm ionic electron. The electrons gets accelerated in between filament and grid by using variable accelerating potential.

A retarding potential of 0.5 volt is connected to plate through ammeter to block the incidence of low energy electrons. The whole glass tube is fitted with mercury vapour at 1mm of Hg.

The graph between accelerating potential difference increasing each step by 0.5 volt is plotted along x-axis and the amount of plate current due to collection of electron is plotted along y-axis as shown in figure.

fig

Franck-Hertz experimental setup:

The equipment that is on the left hand side of Fig. 1 and 2 is the Franck-Hertz control unit. The
equipment that is in the middle of Fig. 1 and 2 is the respective tube. In Fig. 2, the mercury tube is being
encased in a heating oven. For more information about the Franck-Hertz control unit, please refer to
Annex A. For information on the safe usage of the neon and mercury tube, please refer to Annex B.

Explanation:

There is inelastic collision between electron and mercury atom. During inelastic collision electron loses its all kinetic energy mercury. Atom gets excited electron could not reach the plate that's why plate current decreases.

Above 4.9 v potential there elastic collision between electron collides with at 9.8 v p.d a single electron collides with two mercury atom or a single electron losses it's energy completely to single mercury atom.

In this, experiment , Hg atom gets excited and comes to ground state with emission of electron magnetic radiation of wave length 25336\(^A\)

\(\therefore\) Energy o emitted photon= \(\frac{hc}{\lambda}\)

\(=\frac{6.625\times 10^{-34}\times 3\times 10^8}{2536\times 10^{-10}}\)

= 4.9 eV

Calculation:

From the above experimental explanation Frank-Hertz predicted that there is existence of discrete energy levels of Hg atom as well as the existanece of critical potential o f Hg-atom.

Reference:

Reviews of Modern Physics. Lancaster, P.A.: Published for the American Physical Society by the American Institute of Physics, 1952. Print.

Wehr, M. Russell, and James A. Richards. Physics of the Atom. Reading, MA: Addison-Wesley Pub., 1984. Print.

Young, Hugh D., and Roger A. Freedman. University Physics. Boston, MA: Pearson Custom, 2008. Print.

Adhikari, P.B. A Textbook of Physics. 2070 ed. Vol. II. Kathmandu: Sukunda Publication, 2070. Print.