Planck’s Constant by Photoelectric Effect Apparatus

This apparatus helps students observe photoelectric emission in detail.

Demonstrate to students that the emission process depends strongly on the frequency of radiation. The experiment illustrates that for each metal, a critical frequency exists that prevents light of a lower frequency from liberating electrons while light of a higher frequency always does. The emission of electrons occurs within a short time after arrival of the radiation, and the member of electrons is proportional to the intensity of this radiation. Completing this experiment provides the strongest evidence that the electromagnetic field is quantified and that the field consists of quanta of energy (photons): E= hn where n is the frequency of the radiation, and h is the Planck’s constant.

This apparatus includes a vacuum phototube as the photo-sensitive device and a halogen tungsten lamp (12V, 35W) as the light source. With the unit’s optical bench, the light source can be moved to adjust the distance between light source and phototube. The scale length is 400 mm. A focus lens is fixed in the back end, and a drawtube installs the color filters: 635 nm, 570 nm, 540 nm, 500 nm, and 460 nm. Results are displayed on an easy-to-read, seven-segment LED screen. The output is ±15V continuously variable, and the accuracy is ±0.2% 110V ±10%, 60Hz.