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. The configuration of elements as shown on the projection, along with the stopping potential and the photocurrent (amplified by the picoammeter and output to a slave voltmeter). A clearer view of the projected screen and the apparatus is shown in the photographs below.

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. The photoemission material in this device is potassium, and the collector ring is platinum.

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 graph.

SUGGESTIONS: See also P2-02: PHOTOELECTRIC EFFECT IN ZINC, which is simpler but not quantitative. See Question of the Week #250 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 7A10.00)

Here are a number of web site references regarding Planck's constant:

• Measuring Planck's Constant: Experimental setup used at NIST to make a recent measurement of Planck's constant. Reported by by Edwin R. Williams, Richard L. Steiner, David B. Newell, and Paul T. Olsen in Physical Review Letters, v.81, p. 2404, 21 September 1998.
• Measurement of Plank's [sic] Constant Summary of experiment at NIST.
• Planck constant, from the NIST Reference on Constants, Units, and Uncertainty.

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

SETUP TIME: 5 min.