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Wave Particle Duality

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Main experiment for showing light as particles is the photoelectric effect. ... Cathode and anode in a vacuum. ... Sensitive ammeter shows photocurrent ... – PowerPoint PPT presentation

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Title: Wave Particle Duality


1
Wave Particle Duality
  • Advanced Higher Mechanics Topic 9
  • By the Fantastic Four

2
Introduction
  • Wave-Particle duality shows
  • Light can act like a wave and like a particle.
  • Other particles can act as waves
  • Main experiment for showing light as particles is
    the photoelectric effect.

3
The Photoelectric Effect
  • Cathode and anode in a vacuum.
  • Quartz window to illuminate the cathode using an
    ultraviolet light.
  • Sensitive ammeter shows photocurrent
  • Potentiometer provides stopping potential to
    reduce photocurrent to zero.

4
The Photoelectric Effect cont
  • Energy of photoelectrons depends on the frequency
    of the light. Below the threshold frequency, no
    electrons are emitted
  • Hence, light cannot be considered as waves in
    this case but as a stream of particles, called
    photons (1905 Einsteins quantum theory of light)
  • Energy E of a photon
    E hfWhere f frequency of
    beam of light h Plancks constant
    (6.63?1034 Js)
  • This can also be written as E hc
    (vf ?) ?

5
The Photoelectric Effect cont
  • When a photon is absorbed by the cathode, its
    energy is used in exciting an electron.
  • Photoelectron is emitted when the energy is
    sufficient for an electron to escape from an
    atom 
  • Conservation of energy relationship for the
    photoelectric effect
  • hf hf? ½ mv²
  • hf is the energy of incident photon
  • hf? is the work function (min. energy
    required to produce photoelectron)
  • ½ mv² is the kinetic energy of
    photoelectron

6
Compton Scattering
  • Ehf

7
Compton Scattering
  • Conservation of linear momentum

8
Wave-Particle Duality of particles
  • We know light can behave as particles.
  • The equation ? h/? links a property of waves
    (wavelength) with a property of particles
    (momentum).
  • In 1924, Louis de Broglie suggested particles
    have a wavelength.
  • Using the above equation, we can work out the de
    Broglie wavelength of particles.
  • In most cases, this wavelength is VERY small.

9
Example
  • Find the de broglie wavelength of an electron
    travelling at 4 x 105 ms-1.
  • The momentum of the electron is ?e meve 9.11
    x 10-31 x 4 x 105 3.64 x 10-25 kgms-1
  • The de Broglie wavelength is therefore ?e h /
    ?e 6.63 x 10-34 1.82 x 10-9 m
    3.64 x 10-25
  • If the velocity is above about 0.1c, then
    relativistic calculations are needed to be done.
    This is not needed for this course.
  • People like Jannik can also use this to work out
    the wavelength of a bowel of Shreddies.

10
Wave Properties
  • Two properties of waves are
  • Interference
  • If you hit a ball in snooker, the balls dont
    combine to make one big ball, nor do they
    disappear altogether.
  • Diffraction
  • If a train travels through a tunnel, it does not
    spread out when it leaves the tunnel, it
    continues along the track.
  • We have seen these effects before.

11
Electron Diffraction
  • An object like a train has a wavelength many
    times smaller than the width of the tunnel.
  • However, as shown before, electrons have a very
    small wavelength.
  • This wavelength is about the same size as the
    spacing between atoms on a crystalline solid.
  • Therefore, an electron can diffract when passing
    through a crystal.

12
Diffraction Pattern
  • Here is a typical pattern from an electron
    diffracting through a crystalline solid.
  • The different diffraction amounts is due to the
    atomic spacing in the solid and the wavelength of
    the incident beam.

13
Electron Microscope
14
What you NEED to know.
  • The equations, E hf and ? h/?
  • How to use the above equations.
  • Describe and explain the photoelectric effect.
  • Describe and explain electron diffraction.
  • Know that a de Broglie wavelength of a particle
    is extremely small, other than on an atomic or
    sub-atomic level.
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