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This apparatus demonstrates that the energy of a photoelectron depends on the wavelength of the radiation and not its intensity. When the photocathode is irradiated by monochromatic radiation, a potential is applied opposing the energy of the emitted photoelectrons. The voltage required to just stop the current flow is proportional to the energy of the photoelectrons. Plotting this voltage as a function of the reciprocal of the wavelength gives a straight line plot, the slope of which can be used to calculate Planck's constant.

The very small photocurrent is amplified with a FET input operational amplifier located close to the photodiode in the case, and has a minimum detectable photocurrent of about 5 x 10-8 A. The apparatus includes three filters to provide spectral separation.

A small mercury arc with the supplied filters and a He-Ne laser are the best sources for the measurements, but a tungsten lamp can be used with some loss in precision. A separate digital voltmeter is required to measure the stopping potential.

The EP-06 includes the Photoelectric Effect Apparatus, an ES-18 Small Mercury Arc and an ES-19 Lamp Stand. It makes an precise and convenient experiment.