(not on syllabus)
Electrons carry energy and momentum when
they are moving, just like large particles (macro-
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
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
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.
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
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
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
ρ = mv = 3.6 E-23 kgm/s
De Broglie equation λ = h/ρ
= 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.
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
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.
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
This is the opposite of the polarisation of