I.
INTRODUCTION
Photoelectric effect, phenomenon in which electrically charged particles are released from or within a material when it absorbs electromagnetic radiation. The effect is often defined as the ejection of electrons from a metal plate when light falls on it. In a broader definition, the radiant energy may be infrared, visible, or ultraviolet light, X rays, or gamma rays; the material may be a solid, liquid, or gas; and the released particles may be ions (electrically charged atoms or molecules) as well as electrons. The phenomenon was fundamentally significant in the development of modern physics because of the puzzling questions it raised about the nature of light—particle versus wavelike behavior—that were finally resolved by Albert Einstein in 1905. The effect remains important for research in areas from materials science to astrophysics, as well as forming the basis for a variety of useful devices
II.
MAIN BODY
A. BACKGROUND
The photoelectric effect was discovered in 1887 by the German physicist Heinrich Rudolf Hertz. In connection with work on radio waves, Hertz observed that, when ultraviolet light shines on two metal electrodes with a voltage applied across them, the light changes the voltage at which sparking takes place. This relation between light and electricity (hence photoelectric) was clarified in 1902 by another German physicist, Philipp Lenard. He demonstrated that electrically charged particles are liberated from a metal surface when it is illuminated and that these particles are identical to electrons, which had been discovered by the British physicist Joseph John Thomson in 1897.
One inexplicable observation was that the maximum kinetic energy of the released electrons did not vary with the intensity of the light, as expected according to the wave theory, but was proportional instead to the frequency of the light. What the light intensity did determine was the number of electrons released from the metal (measured as an electric current). Another puzzling observation was that there was virtually no time lag between the arrival of radiation and the
emission of electrons.
Consideration of these unexpected behaviours led Albert Einstein to formulate in 1905 a new corpuscular theory of light in which each particle of light, or photon, contains a fixed amount of energy, or quantum, that depends on the light’s frequency. In particular, a photon carries an
energy E equal to hf, where f is the frequency of the light and h is the universal constant that the German physicist Max Planck derived in 1900 to explain the wavelength distribution of blackbody
radiation—that is, the electromagnetic radiation emitted from a hot body. The relationship may also be written in the equivalent form E = hc/λ, where c is the speed of light and λ is its
wavelength, showing that the energy of a photon is inversely proportional to its wavelength. Einstein assumed that a photon would penetrate the material and transfer its energy to an
electron. As the electron moved through the metal at high speed and finally emerged from the material, its kinetic energy would diminish by an amount ϕ called the work function (similar to the
electronic work function), which represents the energy required for the electron to escape the metal. By conservation of energy, this reasoning led Einstein to the photoelectric equation Ek = hf − ϕ, where Ek is the maximum kinetic energy of the ejected electron.
Although Einstein’s model described the emission of electrons from an illuminated plate, his photon hypothesis was sufficiently radical that it was not universally accepted until it received further experimental verification. Further corroboration occurred in 1916 when extremely accurate measurements by the American physicist Robert Millikan verified Einstein’s equation and showed with high precision that the value of Einstein’s constant h was the same as Planck’s constant. Einstein was finally awarded the Nobel Prize for Physics in 1921 for explaining the photoelectric effect.
In 1922 the American physicist Arthur Compton measured the change in wavelength of X rays after they interacted with free electrons, and he showed that the change could be calculated by treating X rays as made of photons. Compton received the 1927 Nobel Prize for Physics for this work. In 1931 the British mathematician Ralph Howard Fowler extended the understanding of photoelectric emission by establishing the relationship between photoelectric current and temperature in metals. Further efforts showed that electromagnetic radiation could also emit electrons in insulators, which do not conduct electricity, and in semiconductors, a variety of insulators that conduct electricity only under certain circumstances.
B. What is Photoelectric effect?
Under the right circumstances light can be used to push electrons, freeing them from the surface of a solid. This process is called the photoelectric effect (or photoelectric emission or photoemission), a material that can exhibit this phenomena is said to be photoemissive, and the ejected electrons are called photoelectrons; but there is nothing that would distinguish them from other electrons. All electrons are identical to one another in mass, charge, spin, and magnetic
moment. In the photoelectric effect, electrons are emitted from solids, liquids or gases when they absorb energy from light. Electrons emitted in this manner may be called photoelectrons.
The photons of a light beam have a characteristic energy proportional to the frequency of the light. In the photoemission process, if an electron within some material absorbs the energy of one photon and acquires more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons excited, but does not increase the energy that each electron possesses. The energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy or frequency of the individual photons. It is an interaction between the incident photon and the outermost electrons.
Electrons can absorb energy from photons when irradiated, but they usually 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 else the energy is re-emitted. If the photon energy is absorbed, some of the energy liberates the electron from the atom, and the rest contributes to the electron's
kinetic energy as a free particle
C. Mathematical Model
The relation between current and applied voltage illustrates the nature of the photoelectric effect. For discussion, a light source illuminates a plate P, and another plate electrode Q collects any emitted electrons. We vary the potential between P and Q and measure the current flowing in the external circuit between the two plates.
If the frequency and the intensity of the incident radiation are fixed, the photoelectric current increases gradually with an increase in positive potential on collector electrode until all the photoelectrons emitted are collected. The photoelectric current attains a saturation value and does not increase further for any increase in the positive potential. The saturation current depends on the intensity of illumination, but not its wavelength.
If we apply a negative potential to plate Q with respect to plate P and gradually increase it, the photoelectric current decreases until it is zero, at a certain negative potential on plate Q. The minimum negative potential given to plate Q at which the photoelectric current becomes zero is called stopping potential or cut off potential
i. For the given frequency of incident radiation, the stopping potential is independent of its intensity.
ii. For a given frequency of the incident radiation, the stopping potential Vo is related to the maximum kinetic energy of the photoelectron that is just stopped from reaching plate Q. If is the mass and is the maximum velocity of photoelectron emitted, then
If qe is the charge on the electron and is the stopping potential, then the work done by the retarding potential in stopping the electron , which gives
The above relation shows that the maximum velocity of the emitted photoelectron is independent of the intensity of the incident light. Hence,
The stopping voltage varies linearly with frequency of light, but depends on the type of material. For any particular material, there is a threshold frequency that must be exceeded, independent of light intensity, to observe any electron emission.
The maximum kinetic energy of an ejected electron is given by
where is the Planck constant and is the frequency of the incident photon. The term = is the work function (sometimes denoted , or ), which gives the minimum energy required to remove a delocalised electron from the surface of the metal. The work function satisfies
where is the threshold frequency for the metal. The maximum kinetic energy of an ejected electron is then
Kinetic energy is positive, so we must have for the photoelectric effect to occur
1.
Diagram of the maximum kinetic energy as a function of the frequency of light on zinc D. Einstein's Equations for the Photoelectric Effect
Einstein's interpretation of the photoelectric effect results in equations which are valid for visible and ultraviolet light:
energy of photon = energy needed to remove an electron + kinetic energy of the emitted electron hν = W + E
where
h is Planck's constant
ν is the frequency of the incident photon
W is the work function, which is the minimum energy required to remove an electron from the surface of a given metal: hν0
E is the maximum kinetic energy of ejected electrons: 1/2 mv2 ν0 is the threshold frequency for the photoelectric effect m is the rest mass of the ejected electron
v is the speed of the ejected electron
No electron will be emitted if the incident photon's energy is less than the work function.
Applying Einstein's special theory of relativity, the relation between energy (E) and momentum (p) of a particle is
E = [(pc)2 + (mc2)2](1/2)
where m is the rest mass of the particle and c is the velocity of light in a vacuum.
The rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light, for a given frequency of incident radiation and metal.
The time between the incidence and emission of a photoelectron is very small, less than 10–9 second.
For a given metal, there is a minimum frequency of incident radiation below which the photoelectic effect will not occur so no photoelectrons can be emitted (threshold frequency).
Above the threshold frequency, the maximum kinetic energy of the emitted photoelectron depends on the frequency of the incident radiation but is independent of its intensity. If the incident light is linearly polarized then the directional distribution of emitted
electrons will peak in the direction of polarization (the direction of the electric field). Planck's Constant
h = 6.63 × 10−34 J s = 4.14 × 10−15 eV s hc = 1.99 × 10−25 J m = 1240 eV nm
III. Application & Usage
A thorough understanding of the photoelectric effect has helped spawn useful applications in many areas of physics, or evenA thorough understanding of the photoelectric effect has helped spawn useful applications in many areas of physics, or even create new areas of study. The most obvious example is probably solar energy, which is produced by photovoltaic cells. These are made of semi-conducting material which produce electricity when exposed to sunlight. An everyday example is a solar powered calculator and a more exotic application would be solar power satellites that orbit around the earth. Engineers are developing new applications for solar energy.
A. Technology based on this principal "electric eye", light meter, movie film audio track
photoconductivity: an increase in the electrical conductivity of a nonmetallic solid when exposed the electromagnetic radiation. The increase in conductivity is due to the addition of free electrons liberated by collision with photons. The rate at which free electrons are generated and the time they over which the remain free determines the amount of the increase.
photovoltaics: the ejected electron travels through the emitting material to enter a solid electrode in contact with the photoemitter (instead of traveling through a vacuum to an anode) leading to the direct conversion of radiant energy to electrical energy
photostatic copying
B. Solar cells
The photo-electric effect may seem like a very easy way to produce electricity from the sun. This is why people choose to make solar panels out of materials like silicon, to generate electricity. In real-life however, the amount of electricity generated is less than expected. This is because not every photon knocks out an electron. Other processes such as reflection or scattering also happen. This means that only a fraction ≈ 10% (depends on the material) of the photons produce
photoelectrons. This drop in efficiency results in a lower current. Much work is being done in industry to improve this efficiency so that the panels can generate as high a current as possible, and create as much electricity as possible from the sun. But even these smaller electrical currents are useful in applications like solar-powered calculators.
C. Image sensors
Video camera tubes in the early days of television used the photoelectric effect, for example, Philo Farnsworth's "Image dissector" used a screen charged by the photoelectric effect to transform an optical image into a scanned electronic signal.[50]
D. Gold-leaf electroscope
The gold leaf electroscope
Gold-leaf electroscopes are designed to detect static electricity. Charge placed on the metal cap spreads to the stem and the gold leaf of the electroscope. Because they then have the same charge, the stem and leaf repel each other. This will cause the leaf to bend away from the stem. The electroscope is an important tool in illustrating the photoelectric effect. For example, if the electroscope is negatively charged throughout, there is an excess of electrons and the leaf is separated from the stem. If high-frequency light shines on the cap, the electroscope discharges and the leaf will fall limp. This is because the frequency of the light shining on the cap is above the cap's threshold frequency. The photons in the light have enough energy to liberate electrons from the cap, reducing its negative charge. This will discharge a negatively charged electroscope and further charge a positive electroscope. However, if the electromagnetic radiation hitting the metal cap does not have a high enough frequency (its frequency is below the threshold value for the cap), then the leaf will never discharge, no matter how long one shines the low-frequency light at the cap.[51]:389-390
E. Photoelectron spectroscopy
Since the energy of the photoelectrons emitted is exactly the energy of the incident photon minus the material's work function or binding energy, the work function of a sample can be determined by bombarding it with a monochromatic X-ray source or UV source, and measuring the kinetic energy distribution of the electrons emitted
Photoelectron spectroscopy is usually done in a high-vacuum environment, since the electrons would be scattered by gas molecules if they were present. However, some companies are now
selling products that allow photoemission in air. The light source can be a laser, a discharge tube, or a synchrotron radiation source.
The concentric hemispherical analyser (CHA) is a typical electron energy analyzer, and uses an electric field to change the directions of incident electrons, depending on their kinetic energies. For every element and core (atomic orbital) there will be a different binding energy. The many electrons created from each of these combinations will show up as spikes in the analyzer output, and these can be used to determine the elemental composition of the sample.
The photomultiplier tube is a highly sensitive extension of the phototube, first developed in the 1930s, which contains a series of metal plates called dynodes. Light striking the cathode releases electrons. These are attracted to the first dynode, where they release additional electrons that strike the second dynode, and so on. After up to 10 dynode stages, the
photocurrent is so enormously amplified that some photomultipliers can virtually detect a single photon. These devices, or solid-state versions of comparable sensitivity, are invaluable in
spectroscopy research, where it is often necessary to measure extremely weak light sources. They are also used in scintillation counters, which contain a material that produces flashes of light when struck by X rays or gamma rays, coupled to a photomultiplier that counts the flashes and measures their intensity. These counters support applications such as identifying particular isotopes for nuclear tracer analysis and detecting X rays used in computerized axial tomography
(CAT) scans to portray a cross section through the body. F. Spacecraft
The photoelectric effect will cause spacecraft exposed to sunlight to develop a positive charge. This can be a major problem, as other parts of the spacecraft in shadow develop a negative charge from nearby plasma, and the imbalance can discharge through delicate electrical components. The static charge created by the photoelectric effect is self-limiting, though, because a more highly charged object gives up its electrons less easily.[53][54]
G. Moon dust
Light from the sun hitting lunar dust causes it to become charged through the photoelectric effect. The charged dust then repels itself and lifts off the surface of the Moon by electrostatic levitation.
[55]
[56] This manifests itself almost like an "atmosphere of dust", visible as a thin haze and blurring of distant features, and visible as a dim glow after the sun has set. This was first photographed by the Surveyor program probes in the 1960s. It is thought that the smallest particles are repelled up to kilometers high, and that the particles move in "fountains" as they charge and discharge.
H. Night vision devices
Photons hitting a thin film of alkali metal or semiconductor material such as gallium arsenide in an image intensifier tube cause the ejection of photoelectrons due to the photoelectric effect. These are accelerated by an electrostatic field where they strike a phosphor coated screen,
converting the electrons back into photons. Intensification of the signal is achieved either through acceleration of the electrons or by increasing the number of electrons through secondary
emissions, such as with a Micro-channel plate. Sometimes a combination of both methods is used. Additional kinetic energy is required to move an electron out of the conduction band and into the vacuum level. This is known as the electron affinity of the photocathode and is another barrier to photoemission other than the forbidden band, explained by the band gap model. Some materials such as Gallium Arsenide have an effective electron affinity that is below the level of the conduction band. In these materials, electrons that move to the conduction band are all of sufficient energy to be emitted from the material and as such, the film that absorbs photons can be quite thick. These materials are known as negative electron affinity materials.
I. Digital Cameras
While this may be the very first time that you have heard of the
photoelectric effect, you have certainly made some use of it many times
in your life. The photoelectric effect is what makes digital cameras work.
Any digital camera contains a grid of little metal “pixels” that become
charged when they are illuminated. This grid sits on what is called a CCD
chip. So when someone takes a picture with a digital camera, the camera
shutter opens and light spills onto the CCD chip. The image is then stored
as the information about the charge on each pixel in the grid. Your eye
contains a similar, but biological version of a CCD chip: the “rods and
cones” of your retina.
J. Other Use
At higher photon energies the analysis of electrons emitted by X rays gives information about electronic transitions among energy states in atoms and molecules. It also contributes to the study of certain nuclear processes, and it plays a role in the chemical analysis of materials, since emitted electrons carry a specific energy that is characteristic of the atomic source. The Compton effect is also used to analyze the properties of materials, and in astronomy it is used to analyze gamma rays that come from cosmic sources.
> Albert Einstein (German-American physicist)
Philipp Lenard (German physicist)
Camera Light meter
Photo voltaic cell
Along with Oxbridge, Imperial and LSE; UCL and Warwick, the remaining members of the G5, are also top Universities to break into the profession from.
After the G5:
Durham/Edinburgh/Nottingham/Bristol
Cass Business School (London)
York/Bath/St Andrews
