Photoelectric Effect
Safety Warnings
Warning: UV Radiation Hazard
Danger: Mercury Vapor Warning
Purpose
UV light from a mercury lamp discharges a negatively charged zinc plate but not a positively charged one, demonstrating the photoelectric effect. Students see that only photons above the threshold frequency eject electrons, providing direct evidence for the quantum nature of light.
Figure 1:
Complete setup showing the mercury lamp directed at the zinc plate attached to the electroscope. The UV light will eject electrons from the negatively charged plate, causing the vane to collapse as charge is lost.
Figure 2:
Glass plate positioned to block UV light while allowing visible light through. This demonstrates that only UV photons have sufficient energy (above zinc's work function) to cause photoemission.
Figure 3:
Complete equipment layout including charging rods (Teflon and Plexiglass), charging fabrics, and glass filters. The various materials allow demonstration of both positive and negative charging, with selective UV filtering.
Equipment
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Demo Apparatus
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