Home > Photoelectric effect
The photoelectric effect is the emission of electrons from a surface (usually metallic) upon exposure to, and absorption of, electromagnetic radiation (such as visible light and ultraviolet radiation) that is above the threshold frequency particular to each type of surface. No electrons are emitted for radiation below the threshold frequency, as they cannot gain sufficient energy to overcome their atomic bonding. The electrons that are emitted are often termed 'Photoelectrons' in many textbooks.The photoelectric effect helped further wave-particle duality, whereby physical systems (such as photons in this case) can display both wave-like and particle-like properties and behaviours, a concept that was used by the creators of quantum mechanics. The photoelectric effect was explained mathematically by Albert Einstein extending the work on quanta developed by Max Planck.
1 History
1.1 Hertz's observations on spark gaps
The first recorded observation of the photoelectric effect was by Heinrich Hertz in 1887 in the journal Annalen Der Physik when he was investigating the production and reception of electromagnetic (EM) waves. His receiver consisted of a coil with a spark gap, whereupon a spark would be seen upon detection of EM waves. He placed the apparatus in a darkened box in order to see the spark better; he observed, however, that the maximum spark length was reduced when in the box. The glass panel placed between the source of EM waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap. When removed, the spark length would increase. He observed no decrease in spark length when he substituted quartz for glass, as quartz does not absorb UV radiation.
Hertz concluded his months of investigation and reported the results obtained. He did not further pursue investigation of this effect, nor did he make any attempt at explaining how the observed phenomenon was brought about.
1.2 JJ Thomson: Electrons
In 1899, Joseph John Thomson investigated ultraviolet light in cathode ray tubeThe cathode ray tube or CRT invented by Karl Ferdinand Braun, is the display device used in most computer displays, video monitors, televisions and oscilloscopes. The CRT developed from Philo Farnsworth's work was used in all television sets until the lats. Influenced by the work of James Clerk MaxwellJames Clerk Maxwell ( June 13, 1831 November 5, 1879) was a Scottish physicist, born in Edinburgh. Maxwell developed a set of equations expressing the basic laws of electricity and magnetism as well as the Maxwell distribution in the kinetic theory of gas, Thomson deduced that cathode rays consisted of negatively charged particles, which he called "corpuscles" (later called "electrons"). In the research, Thomson enclosed a metal plate (i.e., cathode), in a vacuum tube, and exposed it to high frequency radiation. The oscillating electromagnetic fields caused the atoms' field to be resonated and, after reaching a certain amplitude, caused by his model a subatomic "corpuscle" to be emitted, and current to be detected. The current and speed of this current varied with the intensity and color of the radiation. Larger increments of the radiation intensity or frequency of the field would produce more current.
1.3 Von Lenard's observations
In 1902 Philipp von Lenard observed [1] the variation in electron energy with light intensity. He used a powerful electric arc lamp which enabled him to investigate large changes in intensity, and had sufficient power to enable him to investigate the variation of potential with light frequency. His experiment directly measured potentials, not electron kinetic energy: he found the electron energy by relating it to the maximum stopping potential (voltage) in a phototube. He found that the calculated maximum electron kinetic energyKinetic energy (also called vis viva or living force is energy possessed by a body by virtue of its motion. The kinetic energy of a body is equal to the amount of work needed to establish its velocity and rotation, starting from rest. Equations Definition is determined by the frequency of the light. For example, an increase in frequency results in an increase in the maximum kinetic energy calculated for an electron upon liberation - ultraviolet radiation would require a higher applied stopping potential, to stop current in a phototube, than blue light. However Lenard's results were qualitative rather than quantitative because of the difficulty in performing the experiments: the experiments need to be done on freshly cut metal so that the pure metal was observed, but it oxidised in tens of minutes even in the partial vacuums he used. The current emitted by the surface was determined by the light's intensity, or brightness. Doubling the intensity of the light doubled the number of electrons emitted from the surface. Lenard did not know of photons.
