# 1.5 Wave Particle Duality

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2.5 Wave-
particle duality
We Are Learning To
Understand
• Candidates should know that electron diffraction suggests the wave nature
of particles and the photoelectric effect suggests the particle nature of
electromagnetic waves; details of particular methods of particle diffraction
are not expected.
• de Broglie wavelength = h
mv
where mv is the momentum.
Starter

Newton Vs. Huygens
In the 1600s, Christiaan Huygens
and Isaac Newton proposed
competing theories for light's
behavior.

Huygens proposed a wave theory of
Christiaan Huygens         light while Newton's was a
(1629 – 1695)             "corpuscular" (particle) theory of
issues in matching observation.

Newton's prestige helped lend
support to his theory, so for over a
century his theory was dominant.

Sir Isaac Newton (1643 – 1727)
Reflection:- can be
explained by treating light
as a wave or a particle
Interference:
- this can be
explained by the
wave theory of
light (treating light as a
wave)
Refraction of light:-
can be explained
by treating light as
wave or particle
Interference of
light:- can only be
explained by
treating light as a
wave
TOMAS YOUNG 1805   INTERFERENCE EXPERIMENT

constructive
interference

destructive
interference
Diffraction:- can be
only explained by treating
light as a wave
Main

Demonstrate diffraction of light
Light can be diffracted

circular
aperture

light             Light must be a wave
Photoelectric Effect:- can
only be explained if treat light
as a particle (photon)
Main Activity

Particles
Behavior of Electrons
Particle behavior

cathode

anode
Electrons travel in a straight line when
they meet an obstacle they cast a sharp
shadow, no diffraction is observed as
would be with waves.

Also accelerated electrons can be
deflected by magnetic fields and electric
fields, waves are not affected by these
fields.

So electrons are particles, right?
Wave Behaviour of electrons
Test: Can electrons be diffracted?

vacuum
heater

graphite
target

YES, ELECTRONS DO HAVE A WAVE NATURE
Wave behavior of electrons

The particle theory predicts we
should observe a single blurred
spot where the electrons hit the
screen.
In fact we see a interference
pattern which can only be
explained by the wave theory.
The regular atoms in the graphite
diffract the electrons which
interfere to produce regions of
maximum and minimum intensity
TOMAS YOUNG 1805   INTERFERENCE EXPERIMENT

constructive
interference

destructive
interference
Plenary

Duality
Light behaves like water waves in a ripple tank

min

max

min

max

min

max

Light must be a wave
min
Light can be diffracted

circular
aperture

light             Light must be a wave
Photoelectric
LIGHT MUST BE
Emission!        A PARTICLE!
Photon
of Light

potassium metal
PRE 1900 CLASSICAL       1900 PLANCK’S
THEORY            QUANTUM THEORY

ENERGY IS CONTINUOUS    AT THE ATOMIC LEVEL
VARIABLE          ENERGY IS QUANTISED, IT
CANNOT HAVE ANY VALUE
EINSTEIN (1905):   Light comes in packets of energy.

ENERGY OF A PHOTON is

but c = f λ
combining

and

i.e. the wavelength of a photon is Planck’s constant
divided by its momentum, p .
Electrons cast sharp
affected by magnetic
and electric fields,
unlike wave

Electrons must be Particles
Electrons must be Wave-like

Electron
diffraction can
only be explained
if electrons
behave like waves
1923 : Louis de Broglie : “If a photon behaves as
particle with mass, then a particle should have an
associated wavelength given by

where v is the particle’s velocity
Summary

•The wave-like nature of light is observed when
diffraction takes place
•The particle-like nature of light is observed in
the photoelectric effect

•The particle-like nature of electrons is
observed by magnetic and electric deflection
•The wave-like nature of electrons is observed
in electron diffraction
•Particles have a wavelength given by:

According to De Broglie, you have a
wavelength!

What do you notice about it?
What are the implications?

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