Until 1905 not many scientists believed in the subsistence of quanta postulated by Planck. But in 1905 a consecutive phenomenon was explained by that new theory. Albert Einstein did it and the phenomenon is called the photoelectric effect.
In 1887 Hertz discovered the emission of negatively charged particles from metal under the influence of light. Later it appeared that the particles were electrons. Physicists were not surprised by the fact of the emission of particles under the influence of light. They could explain it easily using classical Maxwell equations. Light is an electromagnetic wave and like all such waves it transfers energy. Light can also give that energy away to particles, in this case to electrons knocking them out of the metal. So there would be nothing special about that phenomenon if the classical Maxwell theory gave the right view of its course. But it didn't.
As the classical theory shows, the energy of an electromagnetic wave is connected with its amplitude. What we also know is that amplitude is directly proportional to wave intensity. Hence if illuminating a metal plate with light of higher intensity (brightness), we should expect the knocked out electrons to be of higher energy, which means bigger velocity.
The electromagnetic wave should just transfer more energy to the electrons. The classical theory assumed also a delay of the emission of electrons after illumination. They just should take some time for collecting the energy transferred by the electromagnetic wave before they leave the plate. According to the theory all the light waves, although of different intensities, could knock electrons out of metal; the difference would be only in the length of the delays.
Anyway the experimental researches on the photoelectric effect gave different results to what the theory had assumed before. It emerged that when increasing light intensity only the number of the emitted electrons increases and not their energy. Whereas energy depends on the light frequency (the colour of the light) of light illuminating a plate. The light of higher frequency knocks electrons out with bigger energy. There is also some definite frequency (different for different metals) below which light can't knock out electrons. Even if light intensity is very big but its frequency is lower than the limiting one the photoelectric effect won't occur. In the phenomenon there also doesn't occur the emission delay. All that can't be explained by classical science. The chart shows the interdependence between light frequency and the maximal energy of electrons emitted from metal. It shows the interdependence for three different metals. See that it clearly shows the limiting frequencies- different for different metals].
In 1905 Albert Einstein presented a new theory about how photoelectric effect proceeded. The theory was laid on the ground of the idea of quanta described by Planck five years before. As we said Planck suggested that energy emitted by some heated body was not continuous but emitted in some portions- quanta In 1905 Einstein generalised Planck theory and maintained that light recognised as just a wave before, manifests itself also in a form of some particles called photons. He also suggested was that photons transferred energy in amount that was proportional to the wave frequency. The constant of proportionality in that proportion is Planck's constant h. So once again appears the formula:
Let's now analyse the photoelectric effect using the ideas of quantum theory. Light consists of many photons. The photons collide with a metal plate. Each photon can transfer its energy to one electron. The higher frequency of light, the higher the energy of photons. And the higher is the energy of photons, the higher is the energy of the knocked out electrons. There is some force that bounds electrons with metal. So to knock them out some minimum of energy is needed. If a colliding photon has not enough energy, an electron won't be knocked out. That is why there is some minim frequency, below that no photoelectric effect appears. Increasing light intensity means then increasing the number of photons, not their energy. The more photons there are, the more electrons can be emitted from metal. The whole energy an electron need to leave metal is given to it instantaneously by one photon. That is why there is no delay between the illumination and the emission.
As it was shown here the theory offered by Einstein explained the experiment very well. The theory explained the phenomenon but brought also new questions the hundreds phenomena of nature having been described well by treating light as a wave and yet the idea of light consisting of particles having been formed.
Rudolf Hertz discovered the photoelectric effect in the 20th century.
The details of the photoelectric effect weren't explained by the Maxwell theory.
The higher light frequency, the higher the energy of electrons emitted from metal.
The higher light intensity, the more electrons are knocked out of metal.
Albert Einstein was the one who described the phenomenon correctly.
He assumed that light had corpuscular characteristics - it consisted of photons.
The energy of photons is proportional to the wave frequency.
Light - does it consist of particles or is it a wave?