Franck Hertz Experiment

Named after its creator James Franck and Gustav Hertz.On December 10,1926, Franck and Hertz were awarded the 1925 Nobel Prize in Physics “for their discovery of the laws governing the impact of an electron upon an atom.

Q.What is the use of setting  VG1K at 1.5 volts?

A.It is used to accelerate the electrons emitted from cathode towards anode.

Q.What is the use of setting  VG2A at 7.5 volts?

A.This grid acts as a sort of retarding potential for the electrons if the electrons have more than 7.5 volts then they will pass and reach anode and we will get current otherwise they will return and then re-accelerate towards anode.

Q.What is used to fill the glass tube?Name its suitable replacement.

A.It is filled with mercury vapors because they don’t react with free electrons any inert gas can be its suitable example.e.g-neon,argon etc.

Q.What kind of collision occurs in between electrons and mercury vapors?

A. Both elastic and  inelastic collision occurs.During elastic collision electrons gain energy while during inelastic collision they give all their energy to mercury vapors.

Q.How do we prove the existence of discreet energy levels?

A. As long as the collision between electrons and mercury vapors is elastic the electron will gain energy but after a certain time it reaches a thresh hold and it gains enough energy to ionize Hg vapors by inelastic collision.Thus,it loses its energy and jumps in the immediate next energy level i.e s,p,d,f. When plotting graph it is seen that after an inelastic collision  the graph falls rather abruptly but it should also be noted that every time the graph falls its value will always be greater than the previous fall this along with the rather haphazard  way the graph gets plotted contributes to the idea that electrons exist at different energy levels.

Q.What do you man by Discreet energy levels?

A.A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels. It means that atoms can have only certain definite amount of energy state as light is emitted and absorbed by atoms.

Q.What is Bohr’s theory?

A.Bohr’s theory states that electrons orbiting nucleus can exist only in certain energy levels. A jump from one energy level to another is usually accompanied by absorption or emission of a quantum of radiation.


1.The constant current supply that we give is to the filament for the emission of electrons from the filament and has no effect what so ever on the ampere reading.

2.When using mercury vapors the current drops approximately at an interval of 4.9 eV.

Franck_Hertz Experiment





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