How would you demonstrate the photoelectric effect?
A negatively charged electroscope will discharge if the cap is exposed to high frequency (e.g. ultraviolet) light
Light of high enough frequency (short enough wavelength) will cause electrons to be emitted from the surface of most materials.
If the material is a conductor (e.g. metal) then the electrons can be replaced immediately by a connected circuit to prevent the surface from becoming positively charged and attracting the electrons back to the surface.
The current needed to keep the metal at the same charge is equal to the current generated at the surface by the photoelectric effect.
A gold leaf electroscope can be used to demonstrate this effect. This instrument indicates a charged state when the gold leaf inside it is repelled from the metal stem it is attached to.
In dry air it will stay charged for a long time.
Exposing the metal cap of a negatively charged electroscope to violet or ultraviolet light will discharge it and the gold leaf will return to the discharged position, demonstrating that electrons are being lost from the cap.