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How does one measure the kinetic energy of the ejected electrons resulting from the photoelectric effect?

My understanding is that the KE of the ejected electrons was dependent on the frequency of the incident light, not the intensity. A higher intensity light, however, would [have] ejected MORE electrons; provided that the light's frequency is high enough to overcome the work function of the metal.

Thus, it seems to me that an ammeter would show a higher current either way. A higher frequency light would produce faster electrons and a higher intensity light would produce more electrons. Given that current is a rate of charge (I = dq/dt), how did Einstein know that the frequency was the primary factor for the KE and not the intensity?

This question about the photoelectric effect contains a small misunderstanding of Einstein's prediction. What Einstein said is that the MAXIMUM energy of the ejected electrons should be linearly dependent on the frequency of the light, with the slope of the line equal to Planck's constant.

Light Spectra

The discovery of the photoelectric effect revealed the quantized nature of light.
Image Credit: Christine Nattrass/UT-Knoxville

To measure the maximum energy, you need a voltmeter, not an ammeter. You have to adjust the potential of the surface from which the electrons are being emitted until the flow of electrons stops. The electric potential necessary to prevent any electrons from escaping from the surface, multiplied by the electron's charge, gives the maximum energy of the photoelectrons.

As simple as this sounds, actually carrying out this measurement is rather difficult, and many wrong results were reported between the time Einstein made his prediction, in 1905, and 1916, when Robert A. Millikan published his definitive results showing that Einstein was right.

A nice description of all this, including a critique of previous results, is contained in Millikan's paper in the Physical Review, volume 7, page 355. It is "free to read" on the APS website at As you may know, Einstein received the 1921 Nobel Prize for this work, not for the work on relativity for which he is even more famous.

Answered by:

Alan Chodos, PhD
Associate Executive Officer
American Physical Society

Submitted by:

Matt Davies from The Woodlands, Texas