PURPOSE: Demonstrate the emission of photoelectrons.

DESCRIPTION: A zinc plate connected to an electroscope and charged positive will not discharge under the influence of ultraviolet radiation from the black light. When the plate is charged negative, however, UV light will discharge the plate, as indicated by the electrometer. A 1/8" glass plate inserted into the light from theUV bulb prevents passage of UV and the discharge ceases. Removing the glass plate allows the discharge to continue.

NOTE: This arrangement does not work to show that a positive zinc plate will NOT be discharged by the UV source. If you wish to show the effect with both positive and negative charge on the plate you must use the carbon arc lamp, Demonstration P2-02.

SUGGESTIONS: Zinc plate must be polished before use to clear off oxide and expose zinc surfaces.

REFERENCES: (PIRA 7A10.10)

EQUIPMENT: Black light, glass plate, zinc plate mounted on electroscope, positive and negative charging materials.

SETUP TIME: 5 min.