PURPOSE: Demonstrate the frequency dependence of the photoelectric effect and determine the value of Planck's constant.

DESCRIPTION: Light from a mercury lamp passes through an iris, a lens, and a color filter (centered on either green, blue, or violet mercury lines) and strikes a photoelectric cell. Light incident on the emitter causes photoelectrons to be ejected. The electrons pass through a vacuum to a wire ring collector; the electron current can be measured by a picoammeter. The current can be varied by adjusting the iris.

A variable stopping potential is applied to the cathode ring until the current ceases, providing a measurement of the stopping potential V, which is displayed on a voltmeter. The stopping potential can be measured for the three frequencies of light above by inserting the three filters with the rotating filter inserting gizmo. The stopping potential is observed to increase with the frequency of the incident light.

A plot can be made of the stopping potential as a function of frequency, from which the value of Planck's constant h can be determined: h=e(V2-V1)/(f2-f1), where e is the charge on the electron, V is the stopping potential and f is the frequency of the incident light.

This apparatus includes a transparency of the complete optics and circuitry, on which the current and voltage meters are mounted, so they can be viewed by the entire class. A separate prepared transparency with overhead projector is available for plotting the stopping potential vs. frequency curve.

SUGGESTIONS: See also P2-02: PHOTOELECTRIC EFFECT IN ZINC, which is simpler.


EQUIPMENT: Mounted photoelectric effect demonstration with overhead projector and transparency including meters, graph-plotting blank transparency with overhead projector.

SETUP TIME: 5 min.

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