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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
                           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
                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
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




                          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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