Precision Photoelectric Effect, Daedalon®

The Photoelectric Effect demonstrates that the energy of a photoelectron depends on the wavelength of the incoming radiation and not on its intensity. While monochromatic radiation irradiates the photocathode, a potential is applied opposing the energy of the emitted electrons. 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. Students can use the graph's slope to calculate Planck's constant. A dual-FET ultra-low leakage current input operational amplifier located close to the phototube amplifies the small photocurrent. The amplifier has a minimum detectable current of 1pA. The apparatus includes three color filters to provide spectral separation. A mercury arc with the supplied filters and a He-Ne Laser provides good results, although best results are achieved with multiple laser wavelengths (pointers work well). Students can use a tungsten lamp with less precision. A digital voltmeter or datalogging equipment to measure the stopping potential is required. Specifications: Phototube: R727 mounted inside the amplifier case Amplifier: Ultra low leak current FET op-amp; Dual FET op-amp Current Sensitivity: 1 pA minimum Spectral Separation: provided by red, green, blue gel filters Dimensions: 17 x 9 x 11 cm (3.5 x 6.7 x 4.0 in.)