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Dr. Robert A. Millikan: Fundamental Measurements in Electricity and Radiation, 1937

Photoelectric Effect

Einstein proposed the equation:
E = hf-P
to describe the maximum kinetic energy of each escaping electron in the photoelectric effect. E is the "escape" energy, f is the frequency of incident light, P is Planck's constant and h is the "work function," described by Robert Millikan as "the work necessary to get the electron out of the metal."

Over a ten-year program of experimentation, Robert Millikan devised the apparatus to verify Einstein's theory. Inside an evacuated glass bulb, a plate of an alkali metal, such as lithium, sodium, or potassium, was mounted on a wheel which moved past a scraper knife and then into the path of monochromatic light at various frequencies. Millikan measured the voltage required to prevent the induced current. The graph of incident light frequency vs. voltage was a straight line. Einstein's equation was verified.

Despite his experimental results, Millikan took some time to be completely convinced of Einstein's theory. Confirmation from later experiments on the topic by other scientists eventually changed his mind. The modern theory of light contains both the wave properties and the particle (photon) properties of light.