WARNING: UV light is dangerous to the eyes. Make sure that you don't aim it towards the students, and that you don't look directly at it.

Equipment:

• Braun electroscope - shelf 14A
• zinc plate with banana plug; steel wool - shelf 11E
• Macalaster ultraviolet light source - shelf 12B
• acrylic rod and silk - shelf 14E

Theory: The photons in UV light have an energy which is larger than the work function of electrons in metals. UV light can therefore be used to liberate electrons from a metal, demonstrating the photoelectric effect. Charging by induction is also an important component of this demonstration.

Description: Rub the zinc with the steel wool to remove any oxide, and to expose the metal. Replace the round plate on the top of the electroscope with the zinc. Remove the shield from the UV light source, and shine it on the zinc. Nothing happens. Now bring the positively-charged acrylic rod near the zinc. This will induce a negative charge in the zinc plate, leaving the leaves of the electroscope positively charged. The UV light will bleed off the electrons, and when the rod and light are removed the electroscope will retain a positive charge.

Setup time: 5 minutes.

References: still looking...