CIE A Level Physics复习笔记22.1.3 The Photoelectric Effect: Basics

The Photoelectric Effect: Basics

  • 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

Observing the Photoelectric Effect

  • 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
  • Some notable observations:
    • Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly
    • Using a higher frequency light source does not change the how quickly the gold leaf falls
    • Using a filament light source causes no change in the gold leaf’s position
    • Using a positively charged plate also causes no change in the gold leaf’s position

Typical set-up of the gold leaf electroscope experiment