1905 – photoelectric effect

Maxwell‘s equations showed that light, electricity and magnetism all had similar wave characteristics and suggested that light was a variation of the electro-magnetic wave form. If this was true, light and other electro-magnetic waves impacting metal would cause a flow of electric current in the metal. At the time, scientists were using electrified glass tubes (Geissler tubes) to produce invisible radiation that they called cathode rays, after the negative electrode (cathode) in the tube. The invisible rays were noticed because they caused some substances to glow.

In 1887, Hertz used a primitive radio transmitter and receiver, called a “spark gap generator” to observe a change in the conductivity of a metal plate that was induced by ultraviolet light. This seemed to support the idea that light was a wave.

In 1897, J.J. Thomson measured the mass of cathode rays, showing them to be much lighter than the atom, which at the time was the smallest particle known and thought to be indivisible. This meant that they could be particles which carried electro-magnetic force. He called them “corpuscles”. In 1900, Planck proposed that electromagnetic waves were released in clumps or “quanta”. In 1902, Hertz’s former assistant, Lenard, observed that the energy produced by cathode rays varied with the frequency of the light used instead of the intensity of the light as predicted by Maxwell’s equations. This seemed to fit with Thomson’s ideas and Lenard named the particles “quanta”. In the end, they were named electrons.

In 1905, Einstein extended and combined Planck’s “quanta” concept with the idea of electromagnetic particles and described clumps (quanta) of light particles (photons) causing electrons to be ejected from atoms. Millikan did not accept this idea, but his experiments intended to disprove it, ended up confirming it.

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