Heinrich Hertz made the first observation of the photoelectric effect in 1887. He reported that a spark jumped more readily between two charged spheres if light was shining on them. Further studies were done to learn about the effect observed by Hertz. In 1902, Philipp Lenard showed that the kinetic energy of a photoelectron does not depend on the light intensity. However, it was not until 1905 that Einstein proposed a theory that explained the effect fully. The theory says that electromagnetic radiation is a series of particles, called photons. The photons collide with the electrons on the surface and emit them. This theory ran against the belief that electromagnetic radiation was a wave. Thus, at first it was not recognised as correct. In 1916, Robert Millikan published the results of experiments using a vacuum photo-tube. His work showed that Einstein's photoelectric equation explained the behaviour very accurately. However, Millikan and other scientists were slower to accept Einstein's theory of light quanta. Maxwell's wave theory of electromagnetic radiation cannot explain the photoelectric effect and blackbody radiation. These are explained by quantum mechanics.