Modern Physics

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15 Nov 2009 16:19:05 IST
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plz explain the Davisson-Germer Experiment
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plz explain the Davisson-Germer Experiment

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Dasvidaniya's Avatar

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15 Nov 2009 16:33:01 IST
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Davisson-Germer Experiment

Davisson, C. J., "Are Electrons Waves?," Franklin Institute Journal 205, 597 (1928)

The Davisson-Germer experiment demonstrated the wave nature of the electron, confirming the earlier hypothesis of deBroglie. Putting wave-particle duality on a firm experimental footing, it represented a major step forward in the development of quantum mechanics. The Bragg law for diffraction had been applied to x-ray diffraction, but this was the first application to particle waves.

Davisson and Germer designed and built a vacuum apparatus for the purpose of measuring the energies of electrons scattered from a metal surface. Electrons from a heated filament were accelerated by a voltage and allowed to strike the surface of nickel metal.

The electron beam was directed at the nickel target, which could be rotated to observe angular dependence of the scattered electrons. Their electron detector (called a Faraday box) was mounted on an arc so that it could be rotated to observe electrons at different angles. It was a great surprise to them to find that at certain angles there was a peak in the intensity of the scattered electron beam. This peak indicated wave behavior for the electrons, and could be interpreted by the Bragg law to give values for the lattice spacing in the nickel crystal.

The experimental data above, reproduced above Davisson's article, shows repeated peaks of scattered electron intensity with increasing accelerating voltage. This data was collected at a fixed scattering angle. Using the Bragg law, the deBroglie wavelength expression, and the kinetic energy of the accelerated electrons gives the relationship

In the historical data, an accelerating voltage of 54 volts gave a definite peak at a scattering angle of 50°. The angle theta in the Bragg law corresponding to that scattering angle is 65°, and for that angle the calculated lattice spacing is 0.092 nm. For that lattice spacing and scattering angle, the relationship for wavelength as a function of voltage is empirically


 ( source :- internet)

edison's Avatar

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14 Aug 2010 22:58:15 IST
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The first experimental proof of the wave nature of electron was demonstrated in 1927 by two American physicists C.J Davison and L.H Germer. The basis of their experiment was that since the wavelength of an electron is of the order of spacing of atoms of a crystal, a beam of electrons shoes diffraction effects when incident on a crystal.

Experimental arrangement

The figure shows the experimental setup.

davison germer experiment

Electrons are emitted by a hot filament 'F' by thermionic emission. They are passed between the anode and the cathode, which accelerates the electrons. These accelerated electrons were made to fall on a nickel crystal normally. The beam of electrons is diffracted by the crystal and received at an angle f by a detector D. the intensity of the diffracted electrons is measured by the detector as a function of angle qand also the scattered electron current.

The study of this is represented in the graph.


voltage Vs detector current


As voltages increases first attained a peak and decreases there after. The existence of peak in the graph can be explained as due to constructive interference of waves scattered from atoms in different planes of the crystal. This looks something similar to diffraction pattern of X-rays. Hence, Davison and Germer's experiment stands as an evidence of the wave nature of electrons.

The peak value in the graph can be explained by Bragg's formula

2dSinq = hl

where h is the order of diffraction,

d is the atomic spacing between successive crystal planes and 'q' is the angle at which strong reflection takes place.


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