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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?
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