The photoelectric effect

 Phototube (schematic)

The quartz window admits visible and UV light. The cathode emits photo-electrons which reach the anode unless a small voltage stops even the most energetic.

Plotting the maximum electron energy (eV) as a function of the frequency of the radiation (f) gives a straight line. The slope of the line is Planck's constant (h). The intercept on the vertical axis is defined as the work function (W); the energy needed to remove an electron from a particular metal surface..

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