Physics Demo Manual

Demonstrations are cataloged according to PIRA Bibliography

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Quantum Effects: video - Photoelectric Effect in Zinc; Stopping Potential; Photoconductor (resistor); video - Electron Diffraction; Quantum Levitation (Flux Pinning)
Atomic Physics
Nuclear Physics

Stopping Potential

Purpose:

A quantitative verification of Einstein's famous equation describing the photo-electric effect.

Procedure:

An instruction manual for this h/e experiment is available here. This demo makes use of the PASCO h/e Apparatus AP-9368 and the Mercury Vapor Light Source OS-9286 (replacement lamp). Set up the apparatus as shown in the picture, connecting the output to a voltmeter. The detector consists of a vacuum photodiode connected to a unit gain amplifier. The photodiode and its associated circuitry have a small capacitance. When light falls on the photodiode, the capacitor is charged until the stopping potential is reached and the circuit stabilizes giving a direct reading of the stopping potential for that frequency.

A grating attached to the Hg lamp splits the light into its spectral components. Line up each of the lines with the opening of the detector and read off the stopping potential for each making sure to zero the circuit after each reading. A plot of stopping potential vs frequency will give a straight line, the slope of which gives the value of h/e.

Table of First Order Spectral Lines in Hg
Color Wavelength (nm) Frequency (Hz)

Yellow 578 5.187E+14

Green 546.1 5.490E+14

Blue 435.8 6.879E+14

Violet 404.7 7.409E+14

Ultraviolet 365.5 8.203E+14

Hints:

The lamp should be turned on at the beginning of class as it needs to warm up (a minimum of 5 minutes).
Yellow and Green filters are provided to get rid of higher frequency light, such as any ambient room light. or higher order UV light that overlaps the lower order yellow and green lines. NOTE: We find that with the PASCO filters, you need to use BOTH yellow & green at the same time when measuring the yellow or green stopping potential.
Make sure that the spectral line is aligned well on the detector slit as well as behind the lens on the detector itself.
The cathode of the photodiode is formed by treating an evaporated layer of antimony with cesium vapor at 170 degrees C, resulting in what is believed to be a semiconductor Cs(3)Sb (Cesium-Antimony). This photocathode is characterized by a high sensitivity in the visible spectrum and a low work function. The anode of the photodiode is a nickel alloy. PASCO measured an average work function of 1.36 +/- 0.08 eV.

Reference: Mellissinos, A. C., Experiments in Modern Physics (Academic Press, New York, 1966) pp.18-27