The photoelectric effect occurs when sufficiently energetic photons are incident on a metal, causing electrons to be ejected from the metal. The energy required to eject an electron from a given metal is called the work function Φ of the metal. All additional energy is converted to the kinetic energy of the ejected electron (called a photoelectron).
K = E - Φ
where E is the energy of the absorbed photon:
Work functions for most metals require photons in the UV range to produce a photocurrent of ejected electrons.
This demo shows the particle-like properties of light and is used in photovoltaic cells, camera sensors and light detectors.
In our demonstration an electroscope is negatively charged, causing the hanging rod and the rotating needle to both acquire a negative charge -- this forces the needle to rotate away from the rod. When a UV light is shined on the top plate of the electroscope, electrons are ejected from the metal, bringing it's charge back to neutral, which allows for the needle to return to it's equilibrium position against the hanging rod.
UV-C light (~254nm), electroscope, rabbit fur + PVC pipe (or something similar to deposit negative charges on the electroscope).
The metal plate (the attachment at the top) of the electroscope can become oxidized and may need to be sanded down to get the demo to work. A zinc plate works best if available. If not, aluminum can work, but is less consistent due to oxidation.
UV-C light is dangerous. Don't shine on skin or into eyes.