1.Diffraction of light

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Wave-particle duality of light
Evidence of the wave nature of light

• 1. Diffraction of light
• Light diffracts when it passes the edge of a
  barrier or passes through a slit.

The diffraction of light through a single slit
The diffraction of light around a straight edge
(right)
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• 2. Interference of light
• Light produces an interference pattern when
  passed through a double slit.

Youngs double slit experiment
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Evidence of the particle nature of light

• 1. The photoelectric effect
• The phenomenon could be readily explained using
  the concept of the photon but not the wave theory
  of light.

The photoelectric effect
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• 2. Compton scattering
• When a high-energy EM wave is directed to a free
  electron, the scattered wave has a lower
  frequency.
• Hence a photon carries not only energy but also
  momentum, just as particles do.

Collision between two billiard balls
Compton scattering
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Matter waves
The de Broglie theory

• De Broglie proposed that a matter particle or
  object had an associated matter wave.

• The de Broglie wavelength of a matter particle
  (wave) is
• h the Planck constant
• p momentum of the particle.

Like light, electrons also demonstrate
wave-particle duality.
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The de Broglie wavelength and the wave nature of
matter

• The wave nature of macroscopic objects is
  unobservable because their de Broglie wavelengths
  are too short.

The wavelength of a moving volleyball is much
shorter than any known dimension in daily life.
The wavelength of electrons is comparable to the
interatomic spacing in crystals.
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Electron diffraction the evidence of matter waves
The electron diffraction experiment by George
Thomson
Diffraction rings formed by a beam of electrons
(left) and a beam of X-rays (right) through the
same metal foil
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Electron interference

• The most important evidence of the wave nature of
  light is obtained from Youngs double slit
  experiment.

Interference fringes produced by light

• Similar interference experiments have been
  carried out with electrons as well.

Interference fringes produced by electron
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Implications of the wave-particle duality

• Electrons behave like waves when they pass
  through gaps of small enough sizes, but behave
  like particles when they interact with matter.
• Hence electron exhibits wave-particle duality
  just as light does.
• This also tells us that the microscopic world is
  very different from the macroscopic world we see
  in daily life.

Electrons pass through small gaps in a wave-like
manner but hit the screen in a particle-like
manner.