Diffraction

Electron diffraction.

(not on syllabus)

Electrons carry energy and momentum when they are moving, just like large particles (macro-physical).

The moving electrons also seem to be guided to an interference pattern just like waves of light.

Just like photons of light in the micro-physical world, The particles are guided by ‘matter waves’.

Wave-particle duality was first suggested by Louis de Broglie about a century ago.

An electron diffraction tube works in a similar way to all the other cathode ray tubes we have used.

The electrons are emitted by a cathode and accelerated by a large voltage.

In this tube they hit a carbon target and diffract.

If we bombard a single crystal, such as carbon, with an electron beam the regular arrangement of crystals acts like a diffraction grating. This gives a regular pattern of bright spots as the grating is 2 dimensional.

If we change the P.D. of the accelerator the diffraction pattern changes.

As the pattern of atoms in the carbon target is fixed, as is the geometry of the tube, the only reason for the change is an apparent change in the wavelength of the electrons.

Polycrystalline target.

In reality the carbon target is made up of many crystals. As each crystal will diffract the beam with the same angle, however the crystals are all randomly arranged.

This gives a ring pattern rather than a regular grid.

By measuring the diameter of the rings we can work out the wavelength of the electrons using the diffraction grating formula.

Set the accelerating voltage to 4.5kV, determine the wavelength of the electrons.

We can calculate what the wavelengths should be from the Ep, Ek and momentum of the electron.

Ep = qV = 1.6E-19 x 4500 = 7.2 E-16 J

(Ep = Ek) v = 2Ek/m = 3.9 E7 m/s

= mv = 3.6 E-23 kgm/s

De Broglie equation = h/= 1.84 E-11

Electron diffraction is used in both Transmission electron microscopes and Scanning electron microscopes to analyse crystals.

Similar methods are also used to analyse crystalline samples in X-ray crystallography.

Diffraction of light is used to study the spectra of stars. The spectra produced by different stars can be compared and the red-shift can be determined. This is a key piece of evidence for the big bang.

Polarisation of EM waves.

Transverse waves such as EM waves have oscillations that are at right angles to the direction of propagation.

This can however occur in an infinite number of ways. If we limit the oscillation to just 1 direction we say that the wave is plane polarised.

Polarisation of light.Light can be polarised using polarised filters.

These filters have long organic chains which are regularly spaced, their regular orientation only allows light travelling in one plane to be transmitted.

If another filter is placed at 90 degrees to the first, transmission is completely stopped.

Polarisation filters are used to make skies look more dramatic in photos (they darken the skies as less of the scattered light is recorded. They are also used in sunglasses to reduce glare.

Picket fence model

EM waves are made up of two components the electric field and the magnetic field, these are at right angles to each other.

Aerials for things like TV and radio receive best if they are aligned correctly. If the electric field vector does not match the aerial direction the signal will be weaker.

MagneticElectric

Wave travel

Aerial aligned with incident wave: Strong signal

Aerial not aligned with incident wave: Weaker signal

Polarisation of microwaves

Microwave transmitters are highly polarised.

We can use the 3cm wave set to examine this.

When the metal grate is at 90o to the wave direction it allows the waves to pass.

However if the grate is aligned with the wave, as the energy of the wave is used to generate a current in the bars the wave cannot pass.

This is the opposite of the polarisation of light.