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

Principle

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 ejected electrons depends only on the frequency of the incident light, and is independent of its intensity. This law appears to be in contradiction with the electromagnetic wave theory of the light, but it becomes understandable in the frame of the corpuscular theory of light. The stopping voltage U at different light frequencies is determined by the I /U characteristics of the photocell and plotted over the corresponding light frequency f . Plancks quantum of action is then determined from this graph.

Benefits

• Experience the essence of the Nobel Prize: Planck (1918)
• Using the photocell Planck's "quantum of action" can be measured within few hours
• Modular setup with photocell on an optical bench an a different grating to produce monochromatic light
• Easy Determination of h

Tasks

1. Calculate the light frequency f in dependence on the spectrometer angle
2. Experimentally determine the stopping voltage U for different light frequencies and plot it over light frequency f
3. Calculate Plancks constant from the dependence of the stopping voltage U on the light frequency f

Learning objectives

• Photon energy
• Photon absorption
• External photo effect
• Work function
• Photocell
• Quantum theory
• Plancks constant
• Grating spectrometer