FRANCK – HERTZ EXPERIMENT

The existence of discrete energy levels in an atom was directly verified in by James Franck and Gustav Hertz. They studied the spectrum of mercury vapour when electrons having different kinetic energies passed through the vapour. The electron energy was varied by subjecting the electrons to electric fields of varying strength. The electrons collide with the mercury atoms and can transfer energy to the mercury atoms.

This can only happen when the energy of the electron is higher than the energy difference between an energy level of Hg occupied by an electron and a higher unoccupied level (see Figure). For instance, the difference between an occupied energy level of Hg and a higher unoccupied level is . eV. If an electron of having an energy of .

eV or more passes through mercury, an electron in mercury atom can absorb energy from the bombarding electron and get excited to the higher level [Fig (a)]. The colliding electron’s kinetic energy would reduce by this amount. The excited electron would subsequently fall back to the ground state by emission of radiation [Fig. (b)].

The wavelength of emitted radiation is: . . . hc