2.5 Wave-particle dualityWe 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

Christiaan Huygens(1629 – 1695)

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

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

circular aperture

light

Light can be diffracted

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?

heater

graphite target

vacuum

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

Light must be a wave

max

min

min

max

max

min

min

circular aperture

light

Light can be diffracted

Light must be a wave

potassium metal

Photoelectric Emission!

LIGHT MUST BE A PARTICLE!

Photon of Light

1900 PLANCK’S QUANTUM THEORY

AT THE ATOMIC LEVEL ENERGY IS QUANTISED, IT CANNOT HAVE ANY VALUE

ENERGY IS CONTINUOUS VARIABLE

PRE 1900 CLASSICAL THEORY

EINSTEIN (1905): Light comes in packets of energy.

ENERGY OF A PHOTON is

but c = f λ

hcE

combining

hcE and 2mcE

mch

2mchc

i.e. the wavelength of a photon is Planck’s constant divided by its momentum, p . p

h

Electrons cast sharp shadows and are affected by magnetic and electric fields, unlike wave

Electrons must be Particles

Electron diffraction can

only be explained if electrons

behave like waves

Electrons must be Wave-like

1923 : Louis de Broglie : “If a photon behaves as particle with mass, then a particle should have an associated wavelength given by

ph

mvh

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?