Einstein proposed the equation:
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.
Photograph of Millikan's Certificate of Honorary Membership, (378 k), 1 addl. copy; 5/19/1937
Press release about Millikan's award, 6 pages; 5/19/1937 (1.07 M)