School of Physics Lecture Demonstration Manual

Na-14 Photoelectric effect

Published: Tuesday 05 June 2007 - Updated: Thursday 31 March 2011

Aim

To demonstrate the fundamental aspects of the photoelectric effect. In particular that a minimum quanta of energy is required to emit photoelectrons.

Apparatus

Diagrams

Click on pictures to enlarge.

Photoelectric Setup Pic 1

Photoelectric Setup pic 2

Description

A zinc plate is attached to the electroscope. The ultraviolet source and the 1500 Watt spotlight should be switched on but placed out of sight below the demonstration trolley The electroscope is charged negatively and a torch beam is directed on the zinc plate. The leaves do not collapse. The lecturer may now ask what would be the effect of increasing the number of photons ( the intensity ) and produce the 1500 Watt spotlight from beneath the trolley. When directed onto the zinc plate the leaves again do not collapse. Replacing the spotlight with the ultraviolet light (close to the zinc plate but not touching) the leaves immediately collapse illustrating that a sufficiently energetic photon is required to overcome the work function for the Zinc plate.

Safety notes

Electrical Safety 

Fire/Burn Safety 

Maintenance 

top of page