PURPOSE: Demonstrate the emission of photoelectrons.

DESCRIPTION: In this experiment the arc lamp acts both as a source of ultraviolet radiation for discharging the zinc plate and as a bright light to shadow project the apparatus.

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

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

REFERENCES: (PIRA 7A10.10) Demonstration P2-01: PHOTOELECTRIC EFFECT AND PLANCK'S CONSTANT is a more complicated experiment, but can be easily used to obtain a value for Planck's constant.

EQUIPMENT: Carbon arc lamp, insertable glass plate, zinc plate mounted on electroscope, all on rolling optical table, positive and negative charging materials.

SETUP TIME: 5 min.

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