The Photoelectric Effect: Basics

The photoelectric effect is the phenomenon 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
- 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
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

The motion of the gold leaf demonstrates the photoelectric effect

Make sure to brush up on common misconceptions:

The electrons are already in the metal, they are not produced in any way by the incoming light rays
This experiment proved that light can also behave as a particle as well as a wave
Only one electron can absorb one photon, not multiple

The Photoelectric Effect: Basics (CIE A Level Physics)