History of the Photoelectric Effect

History and expectations as to the photoelectric effect

In 1887, Heinrich Hertz (a German physicist) noticed that ultraviolet light incident on a metal plate could cause sparks. Metals were known to be good conductors of electricity,because the electrons are more able to move. They should be able to be dislodged if energy were added through the incident light. The problem was that different metals required different minimum frequencies of light.

The expectation at the time was that electrons would be emitted for anyfrequency of light, after a delay (for low intensities) during which theelectrons absorbed sufficient energy to escape from the metal surface. Thehigher the intensity the shorter the delay was be as they would absorb energy faster. Thiswas based on the idea that light was a wave continuously delivering energy to theelectrons.

It is important to remember that higher frequency light corresponds to higherenergy.

The next piece of the puzzle came from Philipp Lenard (a Hungarian physicist) in 1902 when he discoveredthat the maximum velocity with which electrons are ejected by ultravioletlight is entirely independent of the intensity of light.

His expectation was that at high intensities the electrons would absorb more energy and so would have a greater velocity.

A paradox existed as the expectations and the observations did not match.

Albert Einstein (a German physicist) solved this paradox by proposingthat light is made up of packetsof energy called quanta (now called photons)which interacted with the electrons in the metal like particles instead of waves.Each incident photon would transfer all its energy to one electron in the metal.

Einstein received the 1921 Nobel Prize for his contribution to understanding thephotoelectric effect. His explanation wasn't very popular and took a while to be accepted,in fact, some scientists at the time felt that is was a big mistake.

In the motivation letter for Einstein to be accepted into the Prussian Academy of Scienceit was specifically mentioned as a mistake: