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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)

Evidence of the wave nature of light:

Wave-particle duality of light

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2.Interference of light– Light produces an interference pattern when passed

through a double slit.

Young’s double slit experiment

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1.The photoelectric effect:– The phenomenon could be readily explained using the

concept of the photon but not the wave theory of light.

Evidence of the particle nature 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 ballsCompton scattering

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• The de Broglie wavelength of a matter particle (wave) is

• h =the Planck constant • p = momentum of the particle.

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

Matter wavesThe de Broglie theory:

Like light, electrons also demonstrate wave-particle duality.

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• 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.

The de Broglie wavelength and the wave nature of matter

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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

Electron diffraction: the evidence of matter waves

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

Electron interference

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.