2.5 Waveparticle 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.
Christiaan Huygens
(1629 – 1695)
Huygens proposed a wave theory of
light while Newton's was a
"corpuscular" (particle) theory of
light. Huygens' theory had some
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
Emission!
LIGHT MUST BE A
PARTICLE!
Photon
of Light
potassium metal
PRE 1900 CLASSICAL
THEORY
1900 PLANCK’S
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
E 
hc

combining
E 
hc
and

hc

 mc
2
E  mc

i.e. the wavelength of a photon is Planck’s constant
divided by its momentum, p .
2
h
mc

h
p
Electrons cast sharp
shadows and are
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
h

 
h
mv

p
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:
What’s your wavelength?
According to De Broglie, you have a
wavelength!
What do you notice about it?
What are the implications?