Light as a Particle - A2 Physics - Turning Points module

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Topic 4 – Light is a Particle

In the exam you are expected to:

  • Discuss the failure of classical wave theory to explain photoelectricity;

  • Explain the significance of Einstein’s explanation of photoelectricity;

  • Explain how de Broglie’s hypothesis was supported by electron diffraction experiments;

  • Use (  (wavelength = planck's constant divided by momentum)

  • Use (

  • Describe the principle of operation of the transmission electron microscope;

  • Describe the principle of operation of the scanning tunnelling electron microscope.


Although light had seemingly been described as a wave, this is not the end of the story. 


The photo-electric effect had initially been described by Hertz in 1887, and developed further by another German, Hallwachs.  A negatively charged zinc plate would emit electrons when exposed to ultra-violet light; but a positively charged plate would not.  Also the plate would not emit any electrons in intense red light, but it would in dim UV light. 


Study of black body radiation provided results that could not be predicted by classical physics.  The formulae worked for long wave radiation, but not short. The German physicist Max Planck (1858 – 1947) tried to explain the observations in terms of classical physics, but could not produce a convincing explanation.  Radically rethinking the problem, he concluded that classical physics does not always apply at the atomic level.

 The classical wave theory could not explain:

  • The instant emission of photons
  • The lack of emission at long wavelengths of light even after an amount of time. In wave theory, electrons could steadily absorb energy.
  • In wave theory, a more intense light wave has more energy and so cannot explain the lack of any emission of photoelectrons.

Einstein proposed that energy was radiated in discrete energy packets called quanta and came up with a formula that seemed to solve the problem.  It was the first formula that used Planck’s constant h (= 6.63 ´10-34 Js).  This was the start of the birth of modern physics.


In 1905 Albert Einstein (1879 – 1955) extended the idea that when a quantum of energy is emitted by an atom, it continues to exist as a concentrated packet of energy.  The energy of the packets (photons) was given by:

E = h f

(it's energy = planck's constant multiplied by its frequency)

A beam of light was to be considered as a stream of particles:

  • The intensity of the light at a distance decreased by the inverse square law.

  • However the energy of each photon was undiminished.


Einstein then went on to state that when a photon collides with an electron, it must

  • either be reflected with no loss of energy,

  • or must lose all its


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