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

Wave Theory

The classical wave theory, published in the 1600s, described light as electromagnetic wave radiation moving in undulating motion through space from sources such as the sun or a light bulb.

Around 1900, Philipp Lenard demonstrated an exception to the accepted theory. He showed that when ultraviolet light was shone on two metal plates set a short distance apart in a vacuum, electric current flowed through the circuit. The ultraviolet light knocked some electrons from one plate; they flew to the other plate and completed the electric circuit. At some increased voltage level the current stopped flowing. This phenomenon is known as the photoelectric effect.

In 1905, Einstein proposed an explanation for this effect built on the work of Max Planck. He stated that light is not distributed in continuous waves, but travels as of a finite number of points of energy (quanta) that move without dividing and can be absorbed or generated only in whole amounts—the behavior of light was more like a stream of particles than a continuous wave. It is a particle which supplies the energy to expel the electron from the metal plate. This theory met with resistance in the scientific community and remained to be experimentally proven.