Planck's "quantum of action" and external photoelectric effect

The photoelectric effect is one key experiment in the development of modern physics. White light from a filament lamp is filtered by a grating spectrometer and illuminates a photocell. The maximum energy of the emitted photoelectrons is determined as a function of the frequency of the incident light. From the linear relationship between stopping voltage and light frequency, Planck's constant h is calculated, confirming Einstein's photon model and the quantum nature of light.