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One of the astounding results of the photoelectric effect experiment was the indepedence of photon electron energy from light intensity. The photoelectron energy rather depends on light frequency.

But, in the first experiment how photoelectron energy was measured?

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Wikipedia page on photoelectric effect has a pretty description about your querry.

In 1902, Lenard observed that the energy of individual emitted electrons increased with the frequency (which is related to the color) of the light Lenard observed the variation in electron energy with light frequency using a powerful electric arc lamp which enabled him to investigate large changes in intensity, and that had sufficient power to enable him to investigate the variation of potential with light frequency. His experiment directly measured potentials, not electron kinetic energy: he found the electron energy by relating it to the maximum stopping potential (voltage) in a phototube.

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When the observer found that the energy of the ejected electron depends upon the frequency of the light instead of the intensity, he measured the potential required for no emission of electron and this was named as "stopping potential". The energy required to stop the moving electron is the work done by the external cell, i.e. $e\Delta V$. This is how the photo electron's energy was determined.

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