Planck’s Constant Experiment
An interesting way to establish Planck’s Constant, the experiment is complete and self-contained, presented in a battery-powered instrument case. A rotary switch selects 1 of the LEDs, viewable through holes in the sloping front panel. There are 5 LEDs of wavelengths 430, 565, 595, 620 and 660mm. By adjusting the potentiometer, the voltage across the selected LED is raised until photon threshold is reached, shown by the LED just beginning to glow. At the threshold it is assumed that all electron energy released in re-combination is converted entirely to photon energy (any higher and phonon energy is is involved). The voltage across the LED is displayed on the built-in digital voltmeter, from which electron energy (and hence photon energy) can be calculated. Knowing the energy and the characteristic wavelength of the LED (indicated on the case), Planck's Constant can be determined.
Please note this requires a PP3 battery.