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
• PVC rod and fur; 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.

Description: Rub the zinc with the steel wool to remove any oxide, and to expose the metal. Remove the round plate from the top of the electroscope, and replace it with the zinc. Then you can show three things:

1. Give the electroscope a negative charge using the PVC rubbed with fur. With the glass shield removed from the UV light source, shine UV light on the zinc. This will discharge the electroscope.
2. Again, give the electroscope a negative charge. Replace the shield on the electroscope, which will absorb most of the UV. Show that the electroscope is not discharged when exposed to this light.
3. Give the electroscope a positive charge, using the acrylic rubbed with silk. Show that the electroscope is not discharged when exposed to the UV light, even if the glass shield is removed from the light.

Setup time: 5 minutes.

References: (available in the demo room)

• Meiners, "Physics Demonstration Experiments", 38-2.1, p. 1169.
• Sutton, "Demonstration Experiments in Physics", A-89, p. 488.
• Carpenter and Minnix, "The Dick and Rae Physics Demo Notebook", S-095.
• Rueckner and Steel, "The Demonstrative Physicist's Companion", QR1.6.