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Wave-Particle Duality
The Legacy of Max Planck
• Before planck, light had been shown to be
wave motion.
• Light does things that only waves do
• E.g. diffraction- this is where light spreads
out when it passes through a single slit
Diffraction at a slit
Only waves behave like this
A diffraction
pattern is
produced when
laser light passes
through a fine slit
Interference
• Waves interfere with each other, they can
reinforce each other or cancel each other
out
Interference
• Waves interfere with each other, they can
reinforce each other or cancel each other
out
After Planck
• Planck had found that light under certain
circumstances behaved as a stream of
particles called photons.
• These particles are as real as electrons.
• The wave and particle nature of light are
both real.
• We need both ideas to explain the
behaviour of light.
• This is called wave-particle duality
De Broglie
• It was not long before De Broglie asked
this question:
• Is it possible that tiny particles like
electrons could have wave behaviour?
• Could they even have a wavelength?
Electrons in atoms
Electrons which orbit nuclei are a particular problem. Theory said they
should not do this. Any accelerating electron should release energy
as photons and should spiral into the nucleus
The electron as a wave
• De Broglie suggested
that the stable orbits
around an atom
corresponded with
whole numbers of
waves.
• The regions between
the orbits could not be
occupied because a
whole number of
waves would not fit.
The De Broglie Wavelength
• Theory led De Broglie
to an equation for the
wavelength of an
electron
λ is the wavelength associated with
the electron
h is planck’s constant
mv is the mass x the velocity of the
electron ( this is called the
momentum of the electron)
h

mv
The equation applies to all particles
but only very small particles like
electrons have measurable
wavelengths.
Electron Diffraction
• If electrons have a wavelength theey
should behave as waves under the right
circumstances.
• For instance they should exhibit effects
like diffraction and interference.
Electron diffraction
This was the first electron
diffraction pattern ever
photographed. The “gaps”
necessary are so small that they
are in fact the gaps between
atoms in a crystal lattice.
The wave nature of electrons is
well established and allows us
to build electron microscopes
Which use the wave nature of the
electron to produce images