When a substance absorbs electromagnetic radiation, electrically charged particles are emitted from or within it, causing the photoelectric effect. The ejection of electrons from a metal plate when light falls on it is a common definition of the effect.
The radiant energy may be infrared, visible, or ultraviolet light, X-rays, or gamma rays; the substance could be a solid, liquid, or gas; and the released particles could be ions (electrically charged atoms or molecules) or electrons.
Because of the puzzling concerns, it raised about the nature of light—particle vs wavelike behaviour—the phenomena were vitally important in the development of modern physics, which were finally resolved by Albert Einstein in 1905. The effect is still essential in a study in fields ranging from materials science to astrophysics, as well as in the development of a range of useful gadgets.
Discovery and Early Work
In 1887, Heinrich Hertz, a German physicist, observed that putting a beam of ultraviolet light upon a metal plate caused it to spark. It wasn’t the emission that took was off guard. Because electrons are more loosely linked to the atoms and can be displaced by a quick burst of incoming energy, metals are known to be good conductors of electricity.
What was perplexing was that different metals required bursts of light with varying minimum frequencies for electron emission to occur, despite the fact that raising the brightness of the light produced more electrons without increasing their energy. Increasing the frequency of the light produced higher-energy electrons without increasing the number of electrons produced.
Hertz found that when ultraviolet light shines on two metal electrodes with a voltage put across them, the voltage at which sparking occurs changes. This was related to his work on radio waves. Philipp Lenard, a German physicist, elucidated the relationship between light and electricity (thus photoelectric) in 1902.
He established that when a metal surface is lighted, electrically charged particles are freed and that these particles are identical to electrons, which were already discovered by British physicist Joseph John Thomson in 1897.
Further research revealed that the photoelectric effect is an interaction between light and matter that classical physics cannot explain, which characterizes light as an electromagnetic wave.
One puzzling finding was that the maximum kinetic energy of the released electrons was proportional to the frequency of the light, rather than the intensity of the light, as predicted by the wave theory.
The number of electrons emitted from the metal was determined by the light intensity (measured as an electric current). Another perplexing finding was that the entrance of radiation and the emission of electrons happened almost simultaneously.
Because of these strange behaviours, Albert Einstein proposed a new corpuscular theory of light in 1905, in which each particle of light, or photon, contains a set quantity of energy, or quantum, that varies with the frequency of the light.
A photon, in particular, has energy (E) equal to (hf), where f is the frequency of light and h is the universal constant developed by German scientist Max Planck in 1900 to explain the wavelength distribution of blackbody radiation, or electromagnetic radiation released from a heated body.
The equation can alternatively be expressed as E =hc/λ, where c is the speed of light and h is the wavelength, indicating that a photon’s energy is inversely proportional to its wavelength.
Einstein predicted that a photon would pass through a substance and transmit its energy to an electron. The kinetic energy of the electron would decrease by an amount called the work function (similar to the electronic work function) as it moved through the metal at high speed and eventually emerged from the material.
The work function represents the energy required for the electron to escape the metal. This argument led Einstein to the photoelectric equation Ek = hf − ϕ, where Ek is the maximum kinetic energy of the ejected electron, and hf is the kinetic energy of the expelled electron.
Einstein released a paper explaining the photoelectric effect in March 1905, while still working as a lowly patent clerk in Switzerland.
Max Planck had addressed the problem of black body radiation five years before by demonstrating that each atom in the cavity’s walls could only absorb or emit radiation in discrete “quanta,” with each quantum’s energy being an integer multiple of its frequency times a new fundamental constant.
Despite the fact that Einstein’s model predicted the emission of electrons from an illuminated plate, his photon idea was so radical that it was not widely accepted until it was confirmed experimentally.
Further confirmation came in 1916 when American physicist Robert Millikan used exceedingly precise measurements to verify Einstein’s equation and prove that the value of Einstein’s constant h was the same as Planck’s constant.
Finally, in 1921, Einstein was awarded the Nobel Prize in Physics for his explanation of the photoelectric effect. Arthur Compton, an American scientist, studied the change in wavelength of X-rays after they interacted with free electrons in 1922 and demonstrated that the change could be estimated by treating X-rays as photons. For this achievement, Compton was awarded the Nobel Prize in Physics in 1927.
Furthermore, Ralph Howard Fowler, a British mathematician, established the link between photoelectric current and temperature in metals in 1931, furthering our understanding of photoelectric emission.
Further research revealed that electromagnetic radiation can emit electrons in insulators (materials that don’t conduct electricity) and semiconductors (a group of insulators that conduct electricity only under certain conditions).
Applications of Photoelectric Effect
While the photoelectric effect’s explanation sounds very theoretical, it has a lot of practical uses. The following are a few:
Originally, photoelectric cells employed a vacuum tube with a cathode to produce electrons and an anode to collect the ensuing current to detect light. These “phototubes” have evolved into semiconductor-based photodiodes, which are now utilized in solar cells and fibre optic telecommunications.
Photomultiplier tubes are similar to phototubes but feature many metal plates known as dynodes. When light strikes the cathodes, electrons are released. The electrons then fall onto the first dynode, which then releases more electrons, which fall onto the second, third, fourth, and so on.
Each dynode amplifies the current until it is powerful enough for the photomultipliers to detect single photons after roughly 10 dynodes. Spectroscopy (which divides light into multiple wavelengths to learn more about the chemical compositions of stars, for example) and computerized axial tomography (CAT) scans, which analyze the body, are two examples of this.
Other uses of Photodiodes and Photomultipliers are
- Imaging technology, such as (older) television camera tubes or image intensifiers
- Studying nuclear practices
- Chemically examining materials based on their emitted electrons
providing theoretical information about how electrons in atoms change between different energy states
The photoelectric effect’s greatest essential application, however, was in initiating the quantum revolution. It prompted scientists to reconsider the nature of light and the structure of atoms in profound ways.