Physics
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Davisson Germer Experiment

Davidson Germer experiment is based on constructive or destructive interference of electrons after reflected from nickel surface.

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Posted by Mahak Jain, 3/11/2021
Davisson Germer Experiment

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Initially atomic models proposed by scientists could only explained the particle nature of electron. All previous models couldn’t able to explain wave nature of electron. C.J.Davisson and L.H. Germer first experimentally verified the wave nature of electrons in 1927. This experiment is famously known as Davisson Germer Experiment. Davisson and Thomson got the Nobel Prize in 1937 for their experimental discovery of diffraction of electrons by crystals. G.P. Thomson, in 1928, observed diffraction effects with beams of electrons scattered by crystals.

Setup of Davisson Germer Experiment

The experimental setup of the Davison Germer Experiment is enclosed within vacuum. Vacuum prevents the scattering of electrons by the medium.

Davisson-Germer electron diffraction arrangement, 

Setup of Davisson Germer Experiment

Davidson germer’s experimental setup consists of :

Electron gun 

Electron gun comprises of a tungsten filament F, coated with barium oxide and heated by a low voltage power supply. When filament F heated, electrons emitted by the filament. Emitted electron accelerates to a desired velocity by applying suitable potential/voltage from a high voltage power supply.

Electrostatic particle accelerator 

Two opposite charged plates positive and negative charged plates accelerate the electrons at a known potential

Collimator  

The electrons which were emitted from the electron gun are made to pass through the axis of a perforated cylinder which consists of fine holes. This will be producing a fine collimated beam. It has a narrow passage for the electrons along its axis. 

Its function is to render a narrow and straight (collimated) beam of electrons ready for acceleration.

A nickel crystal is present in this experiment and the electrons will scatter in various directions since the beams produced from the cylinder are again made to fall on the surface of this nickel crystal. The place of crystal is such that it can rotate about a fixed axis.

Detector 

The electron detector measures the intensity of the electron beam scattered in a given direction.

It is used to capture scattered electron after electron fired on nickel crystal. Detector can be moved in a semicircular shaped arc. 

Working Principle of Davisson Germer Experiment

Davidson Germer experiment is based on constructive or destructive interference of electrons after reflected from nickel surface. 

Working Principle of Davisson Germer Experiment

The waves reflected from two different atomic layers of a nickel crystal will have a fixed phase difference. After reflection, these waves will interfere either constructively or destructively. This constructive and destructive patterns produce a diffraction pattern.

But in Davisson – germers experiment electron was used Instead of waves. Further it was observed that scattered electron also produced a differaction pattern which was thought could only be produced by waves. Therefore this lead to the conclusion that electrons also have wave nature.

Observations Made From Davisson Germer Experiment

  • The experiment was performed by varying the accelerating voltage. Davisson and germer used voltage from 44 V to 68 V in different experiments.
  • A strong peak appeared in the intensity (I) for an accelarating voltage of 54V at a scattering angle θ = 50° for scattering electrons.

The graph between the intensity of scattering and angle of scattering θ

  • The appearance of the peak in a particular direction is due to the constructive interference of electrons scattered from different layers of the regularly spaced atoms of the crystals.
  • From the electron diffraction measurements, the wavelength of matter waves was found to be 0.165 nm.
  • The intensity (I) of the scattered electrons changes by change in the angle of scattering θ. The intensity of the electrons scattered is not continuous. It varies corresponding of a diffraction pattern produced by X-rays.

Relation between Davisson Germer experiment and de Broglie’s Hypothesis

The de broglie hypothesis proposed that all matter exhibits wave nature. 

Now According to de broglie wavelength λ associated with electrons for V = 54 V

     λ = h / P

Here λ : wavelength associated with electrons

       h : plank constant ; 6.67 × 10-34 J×sec

       P : momentum

Now , 

     λ = h / P

Thus, λ = 1.227 / √54

Hence, λ = 0.167 nm 

Thus, there is an excellent agreement between the theoretical value and the experimentally obtained value of de Broglie wavelength.  Thus Davisson germer’s experiment explains wave nature of electron and the de Broglie relation.


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