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
