Devission and Germer Experiment

Description:

Davisson and Germer’s experiment was in the support of de Broglie’s hypothesis.

They demonstrated the diffraction of electron beam similar to diffraction of light waves.  

Figure shows experimental arrangement for electron diffraction.

Apparatus

• Electron gun.
• Nickel crystal.
• Electron detector/collector.

Working

Electron gun has tungsten filament coated with barium oxide for high emission efficiency. When this filament is heated it emits thermal electrons.

The electrons are accelerated by cylindrical shield kept at fixed known high positive voltage (V). The electrons emerge out of shield as fine beam and its energy can be calculated using value of voltage applied.

This beam is made to fall on the surface of nickel crystal.

The electron beam gets reflected after hitting the nickel crystal.

The intensity of reflected electrons in a particular direction is measured by the electron collector, which can be moved on a circular scale.

The collector provides the value of current which is proportionate to the number of electrons incident on it.

The intensity of reflected electron beam is recorded for different angles of deflection (Φ) and different velocities of electrons which is measured by applied voltage (V).

A radial graph is plotted to observe the results of recorded data. It is observed that, current is maximum when deflection angle Φ = 50° and accelerating voltage V = 54V.

This graph shows that electron beam creates diffraction pattern which is a property of wave, so it is confirmed that electron beam has the wave nature.

According to Bragg’s diffraction formula −

Path difference = wavelenght

asinθ = λ

Where,

a = opening for wave to enter.

λ = Wavelength

But in Davisson and Germer’s experiment,

Path difference = 2dsinθ

∴ 2dsinθ = λ

Where,

d = Spacing between atomic planes.

λ = wavelenght

θ = Glancing angle.

For nickel crystal spacing between atomic planes is d = 0.91 × 10^-10

For deflection angle Φ = 50^o

Glancing angle is θ = 65^o as −

glancing angle = 90^o − Φ/2

Therefore,

λ = 2 × 0.91 × 10^-10 × sin(65)

λ = 1.65A

By using de Broglie’s wavelength of electron at accelerating voltage 54V is given by −

This confirms the wave nature electron and proves that materials have dual nature.