Discovery

The discovery and exploration of the photoelectric effect

The part played by Maxwell
In 1865, Maxwell's theory of electromagnetism was published.

This lead to the conclusion that light was an electromagnetic wave.

The part played by Hertz

In 1887, Hertz, tried to generate and detect electromagnetic radiation. In the process he discovered radio waves. His apparatus involved a "receiver" and a "transmitter" of the waves. A spark across a tiny gap(hundredths of a millimeter) indicated the arrival of waves at the receiver. His ability to detect the radiation depended on whether the tiny spark in the receiver was too small to see. In trying to improve the spark's visibility, he made an interesting discovery. He found that the small receiver spark was more vigorous if it was exposed to ultraviolet light from the transmitter spark.

He did not concern himself with why. Nevertheless he had discovered what became known as the photoelectric effect.

The part played by Hallwachs

In 1888, Hallwachs (Germany) investigated the effect that Hertz had noticed.

Hallwachs placed a clean zinc plate on an insulating stand and attached the zinc by a wire to a gold leaf electroscope, which was then charged negatively. The electroscope lost its charge very slowly. However, if ultraviolet light was shone on the zinc plate, the charge leaked away quickly. If the plate was positively charged, there was no fast charge leakage. Thus only negative charges leak away in this effect.

Although Hallwach's experiment clarified the situation, he did not offer any explanation of the effect.

The part played by Thomson

In 1899, Thomson (England) showed that the charges that leaked away due to ultraviolet light had the same charge-to-mass ratio as cathode rays, i.e. they were electrons. The apparatus he used enclosed the metallic surface to be exposed to radiation in a vacuum tube. The cathode rays emitted were identical to those normally released by in other words to make it the cathode in a cathode ray tube. The new feature was that electrons were ejected from the cathode by the ultra-violet radiation, rather than by the strong electric field used previously.

The part played by Lenard

In 1902, Lenard investigated the factors affecting the energy of the electrons emitted by shining light on metals. He placed a second metal plate near the illuminated one and connected it to the illuminated one by a wire. By including a sensitive ammeter, he was able to measure any current produced by the illumination.

To measure the energy of the ejected electrons, Lenard charged the collector plate negatively, to repel the electrons coming towards it. Therefore, only electrons ejected with enough kinetic energy to overcome this repulsion, would contribute to the current. By using powerful and variable light source, Lenard made two surprising discoveries.

There was a well-defined minimum voltage that stopped any electrons reaching the collector. He found that this minimum voltage did not depend at all on the intensity of the light! Increasing the light intensity increased the number of electrons emitted, but did not affect the energies of the emitted electrons.
When he used light of different colours, he found that the maximum energy of the ejected electrons depended on the colour (and hence on the frequency) ---the higher the frequency of the light, the more energetic the electrons.

The part played by Einstein

To understand the part played by Einstein it is best to study the Theory section of this website first. However, before delving into the theory, you may wish to read short biographies of Einstein or the other scientists involved, or view a Timeline of events.