CIE A Level Physics复习笔记22.1.3 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