The Davisson-Germer Experiment

The experiment gave the evidence of wave nature possession by materialistic particle(electron) for the first time.

The arrangement of equipment used for the experiment is as follows

                      Procedure:

An electron gun is used in order to produce electrons. The electrons so produced are accelerated by applying a high potential towards the target crystal, in this case the target crystal is nicker. The accelerated electron beam is made into fine beam by passing it through a Collimator ‘c’.

The crystal is mounted on an arrangement which could be rotated in different directions perpendicular to the plane of diagram.

The electrons are scattered in all directions by atomic planes of crystal.

The intensity of electron beam ( no. Of electrons) scattered in a particular direction is measured by electron collector which can be rotated about the same axis as target crystal.

The collector is connected to a sensitive Galvanometer whose deflection is proportional to intensity of electron beam entering collector . The electron collector is also called Faraday cylinder.

A retarding potential is applied to Faraday cylinder such that only fast electrons can reach it and secondary electrons emitted from crystals are stopped.

A graph is then plotted between galvanometer current against angle ‘θ' between incident beam and diffracted beam i.e, beam entering Faraday cylinder.



In the investigation , the electron beam accelerated by 54V and at an angle of 50 between incident and diffracted beam , a sharp maximum has occurred in electron distribution.

The incident beam and diffracted beam in this experiment make an angle of 65⁰   with Braggs plane. 

For a 54 V electron , the de-broglie wavelength associated with the electron is given by

ƛ = 12.25/√V = 12.25/√54 A⁰ = 1.66 A⁰ 

Now from Bragg’s equation for maxima in diffraction pattern for same energy electrons

2d sinθ' = nƛ; 2*0.91*10^-10*sin 65⁰ = 1*ƛ;

ƛ = 1.65 A⁰ 

Thus, both theoretical and experimental values are in excellent agreement.

Thus Davission - Germer experiment provides a direct verification of de-broglie hypothesis of wave nature of moving particles.

GP THOMSON EFFECT - Experimental Verification of Wave nature of Matter

G P  Thomson has performed experiments with electrons accelerated from 10000 to 50000 volts.

The high energy beam of electrons is produced by a cathode ‘C’.

The experimental arrangement is as shown below:

The electron beam is excited with potential upto a maximum of 50,000 volts. A fine beam is obtained by passing it through slit or diaphragm ‘S’.

The accelerating fine beam of electrons now falls on thin gold or Aluminum film (order of 10^-6 cm thickness).

The photograph of beam from foil is recorded on photographic plate ‘P’.

After developing the plate, a symmetric pattern consisting of concentric rings about a central spot is obtained. This pattern resembles that of X- rays.

To know that this pattern is due to electrons or due to x-rays generated by electrons in their passage through foil, cathode rays in discharge tube are deflected by magnetic field.

It was observed that beam shifts correspondingly showing there by that pattern is produced by electrons and not by x-rays.  (i.e X – ray pattern is not affected by electric and magnetic fields).

As diffraction pattern can only be produced by waves and not by particles, so Thomson concluded that electrons behave like wave.

Thus, Thomson experiment clearly demonstrated the existence of matter waves.

The Bloch Theorem