Edexcel International A Level Physics

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2.27 The Photoelectric Effect

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The Photoelectric Effect

  • The photoelectric effect is the phenomena in which electrons are emitted from the surface of a metal upon the absorption of electromagnetic radiation
  • Electrons removed from a metal in this manner are known as photoelectrons
  • The photoelectric effect provides important evidence that light is quantised, or carried in discrete packets
    • This is shown by the fact each electron can absorb only a single photon
    • This means only the frequencies of light above a threshold frequency will emit a photoelectron

Photoelectrons are emitted from the surface of metal when light shines onto it

  • The photoelectric effect can be observed on a gold leaf electroscope
  • A plate of metal, usually zinc, is attached to a gold leaf, which initially has a negative charge, causing it to be repelled by a central negatively charged rod
    • This causes negative charge, or electrons, to build up on the zinc plate

  • UV light is shone onto the metal plate, leading to the emission of photoelectrons
  • This causes the extra electrons on the central rod and gold leaf to be removed, so, the gold leaf begins to fall back towards the central rod
    • This is because they become less negatively charged, and hence repel less

Typical set-up of the gold leaf electroscope experiment

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