Photoelectric effect

The photoelectric effect is the phenomenon whereby electrons are emitted 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 the electrons cannot gain sufficient energy to overcome their atomic bonding.

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 utilising the work in quantum mechanics developed by such people as Max Planck.

Table of contents
1 History
2 Explanations
3 Common Uses
4 See also
5 External links and references

History

Hertz's Observations

The first recorded observation of the photoelectric effect was by Heinrich Hertz in 1887 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 decrease. 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. Hertz did not futher pursue the investigation of this effect, nor did he make any attempt at explaining how the observed phenomena were brought about.

Electrons

In 1899, Joseph John Thomson investigates ultraviolet light in cathode ray tubes. Influenced by the work of James Clerk Maxwell, Thomson deduced that cathode rays existed of negatively charged particles, which he called "corpuscles" (later called "electrons"). In the research, Thomson encloses 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 certian amplitude, cause a subatomic "corpuscle" to be emitted. The number and speed of these vary with the intensity and color of the radiation. Larger increments of the radiation intensity or frequency of the field would produce more electrons.

Radiant Energy

In 1901 on November 5, Nikola Tesla received the patent US685957 (Apparatus for the Utilization of Radiant Energy) that describes radiation charging and discharging conductors (ex., a metal plate) by "radiant energy". Tesla used this effect to charge a capacitor with energy by mean of a conductive plate (eg., a solar cell precursor). The radiant energy throws off with great velocity minute particles (i.e., "electrons") which are strongly electrified. The patent specified that the radiation (or radiant energy) include many different forms. These devices have been referred to as "Photoelectric alternating current stepping motors".

In practice, a polished metal plate in radiant energy (ex. sunlight) will charge positively as electrons are emmitted by the plates. As the plate charges positively, electrons form a electrostatic force on the plate (because of surface emmissions of the photoelectrons), and "drain" any negativlely charged capacitors. As the rays or radiation falling on the insulated-conductor (and which is connected to a condenser (i.e., a capacitor)), the condenser will indefinitely charge electrically.

Light Quanta

Although the effect itself had been described earlier by Nikola Tesla in the patent US685957, Albert Einstein's experimental demonstration helped him win the Nobel Prize of 1921. Einstein's mathematical description in 1905 on how it was caused by absorption of photons, or quanta of light, in the interaction of light with the electrons in the substance) was contained in the paper named "On a Heuristic Viewpoint Concerning the Production and Transformation of Light". This paper proposed the simple description of "light quanta" (later called "photons") and showed how they could be used to explain such phenomena as the photoelectric effect. The simple explanation by Einstein in terms of absorption of single quanta of light explained the features of the phenomenon and helped explained the characteristic energy.

The idea of light quanta was motivated by Max Planck's published law of blackbody radiation ("On the Law of Distribution of Energy in the Normal Spectrum". Annalen der Physik 4 (1901)) by assuming that luminous energy could only be absorbed or emitted in discrete amounts, called quanta. Einstein showed that, by assuming that light actually consisted of discrete packets, he could explain the photoelectric effect. The idea of light quanta contradicted the wave theory of light that followed naturally from James Clerk Maxwell's equations for electromagnetic behavior and, more generally, the assumption of infinite divisibility of energy in physical systems. Even after experiments showed that Einstein's equations for the photoelectric effect were accurate, his explanation was not universally accepted.

The photons of the light beam have a characteristic energy given by the wavelength of the light. In the photoemission process, if an electron absorbs the energy of one photon and has more energy than the work function, it is ejected from the material. If the photon energy is too low, however, the electron is unable to escape the surface of the material. Increasing the intensity of the light beam does not change the energy of the constituent photons, only their number, and thus the energy of the emitted electrons does not depend on the intensity of the incoming light. It was for this insight that Einstein won his only Nobel Prize.

Explanations

The Photoelectric Effect help propel the then-emerging concept of the dual nature of light (light exhibits characteristics of waves and particles at different times). It was difficult to understand in terms of the classic wave description of light, as the energy of the emitted electrons did not depend on the intensity of the incident radiation. Classical theory predicted that the electrons could 'gather up' energy over a period of time, and then be emitted, which did not occur.

For a given surface, there is a minimum frequency (or maximum wavelength, since frequency and wavelength are inversely proportional), whereby incident radiation with a lower frequency than the threshold, and hence lower energy than the minimum required, did not cause electrons to be emitted, regardless of intensity.

Einstein explained this by conceptualising light as being broken into packets (quanta) of energy. Electrons can absorb energy from photons when irradiated, but they follow an "all or nothing" principle. All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or the energy is re-emitted. If the photon is absorbed, some of the energy is used to liberate it from the atom, and the rest contributes to the electron's kinetic (moving) energy as a free particle.

Philipp von Lenard observed the following:

  • The maximum kinetic energy is determined by the photon's energy (and hence frequency/wavelength). For example, an increase in frequency results in a proportional increase in the maximum kinetic energy possessed by the electron upon liberation - ultraviolet radiation would give more energy to the electrons than blue light. The maximum kinetic energy corresponds to the potential difference (voltage) produced if the electrons were harnessed by a collector, and put to work in a circuit.
  • The number of electrons emitted by the surface is determined by the number of photons (hence the light's intensity, or brightness). For example, doubling the intensity of the light doubles the number of electrons emitted from the surface. The number of electrons corresponds to the current produced by a photocell.

Equations

In analysing the photoelectric effect quantitatively, the following equations are used:

Energy of photon = Energy needed to remove an electron + Kinetic energy of the emitted electron

Or hf = hf0 + ½ mvmax2

Or hf = ø + Ek

h - Planck's constant
f0 - threshold frequency for the photoelectric effect to occur
ø - work function, or minimum energy required to remove electron from atomic binding
Ek - maximum kinetic energy observed

  • Note: An electron will be emitted if hf > ø.

When this equation is not observed to be true, it may be because when given an excess amount of energy to the body, some energy is absorbed as heat or emitted as radiation, as no system is perfectly efficient.

Common Uses

The photoelectric effect is used for such things as solar power by mounting arrays of solar cells which generate direct current from sunlight, and light-sensitive diodes.

Electroscopes

Electroscopes are fork-shaped, hinged metallic leaves placed in a vacuum jar, partially exposed to the outside environment. When an electroscope is charged positively or negatively, the two leaves separate, as charge distributes evenly along the leaves causing repulsion between two like poles. When ultraviolet radiation (or any radiation above threshold frequency) is shone onto the metallic outside of the electroscope, the negatively charged one will discharge and collapse, while nothing will happen to the positively charged one. The reason is that electrons will be liberated from the negatively charged one, gradually making it neutral, while liberating electrons from the positively charged one will make it even more positive, keeping the leaves apart.

See also

Electronics Physics People Lists Other

External links and references

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