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Photoelectric Effect

Photoelectric effect

When visible light, X rays, gamma rays, or other forms of electromagnetic radiation are shined on certain kinds of matter, electrons are ejected. That phenomenon is known as the photoelectric effect. The photoelectric effect was discovered by German physicist Heinrich Hertz (18571894) in 1887. You can imagine the effect as follows: Suppose that a metal plate is attached by two wires to a galvanometer. (A galvanometer is an instrument for measuring the flow of electric current.) If light of the correct color is shined on the metal plate, the galvanometer may register a current. That reading indicates that electrons have been ejected from the metal plate. Those electrons then flow through the external wires and the galvanometer, providing the observed reading.

Photoelectric theory

The photoelectric effect is important in history because it caused scientists to think about light and other forms of electromagnetic radiation in a different way. The peculiar thing about the photoelectric effect is the relationship between the intensity of the light shined on a piece of metal and the amount of electric current produced.

Words to Know

Anode: The electrode in an electrochemical cell at which electrons are given up to a reaction.

Cathode: The electrode in an electrochemical cell at which electrons are taken up from a reaction.

Electrode: A material that will conduct an electrical current, usually a metal, used to carry electrons into or out of an electrochemical cell.

Electromagnetic radiation: Radiation (energy in the form of waves or subatomic particles) that transmits energy through the interaction of electricity and magnetism.

Frequency: The number of times a wave passes a given point in space per unit of time (as per second).

Photocell: A vacuum tube in which electric current flows when light strikes the photosensitive (or light sensitive) cathode.

Photon: A particle of light whose energy depends on its frequency.

Solar cell: A device constructed from specially prepared silicon that converts radiant energy (light) into electrical energy.

To scientists, it seemed reasonable that you could make a stronger current flow if you shined a brighter light on the metal. More (or brighter) light should produce more electric currentor so everyone thought. But that isn't the case. For example, shining a very weak red light and a very strong red light on a piece of metal produces the same results. What does make a difference, though, is the color of the light used.

One way that scientists express the color of light is by specifying its frequency. The frequency of light and other forms of electromagnetic radiation is the number of times per second that light (or radiation) waves pass a given point. What scientists discovered was that light of some frequencies can produce an electric current, while light of other frequencies cannot.

Einstein's explanation. This strange observation was explained in 1905 by German-born American physicist Albert Einstein (18791955). Einstein hypothesized that light travels in the form of tiny packets of energy, now called photons. The amount of energy in each photon is equal to the frequency of light (ν) multiplied by a constant known as Planck's constant (), or ν.

Einstein further suggested that electrons can be ejected from a material if they absorb exactly one photon of light, not a half photon, or a third photon, or some other fractional amount. Green light might not be effective in causing the photoelectric effect with some metals, Einstein said, because a photon of green light might not have exactly the right energy to eject an electron. But a photon of red light might have just the right amount of energy.

Einstein's explanation of the photoelectric effect was very important because it provided scientists with an alternative method of describing light. For centuries, researchers had thought of light as a form of energy that travels in waves. And that explanation works for many phenomena. But it does not work for phenomena such as the photoelectric effect and certain other properties of light.

Today, scientists have two different but complementary ways of describing light. In some cases, they say, it behaves like a wave. But in other cases, it behaves like a stream of particlesa stream of photons.

Applications

Two of the most important applications of the photoelectric effect are the photoelectric cell (or photocell) and solar cells. A photocell usually consists of a vacuum tube with two electrodes. A vacuum tube is a glass tube from which almost all of the air has been removed. The electrodes are two metal plates or wires. One electrode in a photocell consists of a metal (the cathode) that will emit electrons when exposed to light. The other electrode (the anode) is given a positive electric charge compared to the cathode. When light shines on the cathode, electrons are emitted and then attracted to the anode. An electron current flows in the tube from cathode to anode. The current can be used to turn on a motor, to open a door, or to ring a bell in an alarm system. The system can be made to respond to light, as described above, or it can be sensitive to the removal of light.

Photocells are commonly used in factories. Items on a conveyor belt pass between a beam of light and a photocell. As each item passes the beam, it interrupts the light, the current in the photocell stops, and a counter is turned on. With this method, the exact number of items leaving the factory can be counted. Photocells are also installed on light poles to turn street lights on and off at dusk and dawn. In addition, photocells are used as exposure meters in cameras. They measure the exact amount of light entering a camera, allowing a photographer to adjust the camera's lens to the correct setting.

Solar cells are devices for converting radiant energy (light) into electrical energy. They are usually made of specially prepared silicon that emits electrons when exposed to light. When a solar cell is exposed to sunlight, electrons emitted by silicon flow through external wires as a current.

Individual solar cells produce voltages of about 0.6 volts each. In most practical applications, higher voltages and large currents can be obtained by connecting many solar cells together. Electricity from solar cells is still quite expensive, but these cells remain very useful for providing small amounts of electricity in remote locations where other sources are not available. As the cost of producing solar cells is reduced, however, they will begin to be used for the production of large amounts of electricity for commercial use.

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photoelectric effect

photoelectric effect, emission of electrons by substances, especially metals, when light falls on their surfaces. The effect was discovered by H. R. Hertz in 1887. The failure of the classical theory of electromagnetic radiation to explain it helped lead to the development of the quantum theory. According to classical theory, when light, thought to be composed of waves, strikes substances, the energy of the liberated electrons ought to be proportional to the intensity of light. Experiments showed that, although the electron current produced depends upon the intensity of the light, the maximum energy of the electrons was not dependent on the intensity. Moreover, classical theory predicted that the photoelectric current should not depend on the frequency of the light and that there should be a time lag between the reception of light on the surface and the emission of the electrons. Neither of these predictions was borne out by experiment. In 1905, Albert Einstein published a theory that successfully explained the photoelectric effect. It was closely related to Planck's theory of blackbody radiation announced in 1900. According to Einstein's theory, the incident light is composed of discrete particles of energy, or quanta, called photons, the energy of each photon being proportional to its frequency according to the equation E=hυ, where E is the energy, υ is the frequency, and h is Planck's constant. Each photoelectron ejected is the result of the absorption of one photon. The maximum kinetic energy, KE, that any photoelectron can possess is given by KE = hυ-W, where W is the work function, i.e., the energy required to free an electron from the material, varying with the particular material. The effect has a number of practical applications, most based on the photoelectric cell.

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photoelectric effect

photoelectric effect Liberation of electrons from the surface of a material when light, ultraviolet radiation, X-rays, or gamma rays fall on it. The effect can be explained only by the quantum theory: photons in the radiation are absorbed by atoms in the substance and enable electrons to escape by transferring energy to them.

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Photoelectric Effect

Photoelectric Effect

History

The Einstein photoelectric theory

Applications

Resources

The photoelectric effect is the process in which electromagnetic radiation such as visible light, x rays, or gamma rays strike matter and cause an electron to be ejected. The ejected electron is called a photoelectron.

History

The photoelectric effect was discovered by German physicist Heinrich Rudolph Hertz (1857 1894) in 1897 while performing experiments that led to the discovery of electromagnetic waves. Since this was just about the time that the electron itself was first identified, the phenomenon was not really understood. It soon became clear in the next few years that the particles emitted in the photoelectric effect were indeed electrons. The number of electrons emitted depended on the intensity of the light but the energy of the photoelectrons did not. No matter how weak the light source was made the maximum kinetic energy of these electrons stayed the same. The energy however was found to be directly proportional to the frequency of the light. The other perplexing fact was that the photoelectrons seemed to be emitted instantaneously when the light was turned on. These facts were impossible to explain with the then current wave theory of light. If the light were bright enough it seemed reasonable, given enough time, that an electron in an atom might acquire enough energy to escape regardless of the frequency.

The answer was finally provided in 1905 by GermanAmerican physicist Albert Einstein (1879 1955) who suggested that light, at least sometimes, should be considered to be composed of small bundles of energy or particles called photons. This approach had been used a few years earlier by Max Planck in his successful explanation of black body radiation. In 1907 Einstein was awarded the Nobel Prize in physics for his explanation of the photoelectric effect.

The Einstein photoelectric theory

Einsteins explanation of the photoelectric effect was very simple. He assumed that the kinetic energy of the ejected electron was equal to the energy of the incident photon minus the energy required to remove the electron from the material, which is called the work function. Thus the photon hits a surface, gives nearly all its energy to an electron and the electron is ejected with that energy less whatever energy is required to get it out of the atom and away from the surface. The energy (E) of a photon is given by E = hγ = hc/λ where h is Plancks constant (6.62617 x 10-34 Joules-second), γ is the frequency of the photon, λ is the wavelength of the photon, and c is the speed of light (2.99792 X 108 meters-second-1). This applies not only to light but also to x rays and gamma rays. Thus, the shorter the wavelength the more energetic the photon. Einsteins explanation of the photoelectric effect helped with the development of quantum theory, the description of matter and energy in the universe.

This energy applies not only to light but also to x rays and gamma rays. Thus, the shorter the wavelength of the electromagnetic radiation, the more energetic becomes the photon. Einsteins explanation of the photoelectric effect helped with the development of quantum theory, the description of matter and energy in the universe.

Many of the properties of light such as interference and diffraction can be explained most naturally by a wave theory while others, like the photoelectric effect, can only be explained by a particle theory. This peculiar fact is often referred to as wave-particle duality and can only be understood using quantum theory, which must be used to explain what happens on an atomic scale and that provides a unified description of both processes.

Applications

The photoelectric effect has many practical applications which include the photocell, photoconductive devices and solar cells. A photocell is usually a vacuum tube with two electrodes. One is a photosensitive cathode, which emits electrons when exposed to light and the other is an anode, which is maintained at a positive voltage with respect to the cathode. Thus, when light shines on the cathode, electrons are attracted to the anode and an electron current flows in the tube from cathode to anode. The current can be used to operate a relay, which might turn a motor on to open a door or ring a bell in an alarm system. The system can be made to be responsive to light, as described above, or sensitive to the removal of light as when a beam of light incident on the cathode is interrupted, causing the current to stop. Photocells are also useful as exposure

KEY TERMS

Photocell A vacuum tube in which electric current will flow when light strikes the photosensitive cathode.

Photoconductivity The substantial increase in conductivity acquired by certain materials when exposed to light.

Photoelectric effect The ejection of an electron from a material substance by electromagnetic radiation incident on that substance.

Photoelectron Name given the electron ejected in the photoelectric effect.

Solar cell A device by which sunlight is converted into electricity.

Work function The amount of energy required to just remove a photoelectron from a surface. This is different for different materials.

meters for cameras in which case the current in the tube would be measured directly on a sensitive meter.

Closely related to the photoelectric effect is the photoconductive effect which is the increase in electrical conductivity of certain non metallic materials such as cadmium sulfide when exposed to light. This effect can be quite large so that a very small current in a device suddenly becomes quite large when exposed to light. Thus photoconductive devices have many of the same uses as photocells.

Solar cells, usually made from specially prepared silicon, act like a battery when exposed to light. Individual solar cells produce voltages of about 0.6 volts but higher voltages and large currents can be obtained by appropriately connecting many solar cells together. Electricity from solar cells is still quite expensive but they are very useful for providing small amounts of electricity in remote locations where other sources are not available. It is likely however that as the cost of producing solar cells is reduced they will begin to be used to produce large amounts of electricity for commercial use.

Resources

BOOKS

Brooker, Geoffrey. Modern Classical Optics. Oxford, UK: Oxford University Press, 2003.

Chartier, Germain. Introduction to Optics. New York: Springer, 2005.

Griffith, W. Thomas. The Physics of Everyday Phenomena: A Conceptual Introduction to Physics. Boston, MA: McGraw-Hill, 2004.

Menn, Naftaly. Practical Optics. Amsterdam, Netherlands, and Boston, MA: Elsevier Academic Press, 2004.

Saslow, Wayne M. Electricity, Magnetism, and Light. Amsterdam, Netherlands, and Boston, MA: Academic Press, 2002.

Serway, Raymond, Jerry S. Faughn, and Clement J. Moses. College Physics. 6th ed. Pacific Grove, CA: Brooks/ Cole, 2002.

Young, Hugh D. Sears and Zemanskys University Physics. San Francisco, CA: Pearson Addison Wesley, 2004.

Robert Stearns

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Photoelectric Effect

Photoelectric effect

The process in which visible light , x rays, or gamma rays incident on matter cause an electron to be ejected. The ejected electron is called a photoelectron.


History

The photoelectric effect was discovered by Heinrich Hertz in 1897 while performing experiments that led to the discovery of electromagnetic waves. Since this was just about the time that the electron itself was first identified the phenomenon was not really understood. It soon became clear in the next few years that the particles emitted in the photoelectric effect were indeed electrons. The number of electrons emitted depended on the intensity of the light but the energy of the photoelectrons did not. No matter how weak the light source was made the maximum kinetic energy of these electrons stayed the same. The energy however was found to be directly proportional to the frequency of the light. The other perplexing fact was that the photoelectrons seemed to be emitted instantaneously when the light was turned on. These facts were impossible to explain with the then current wave theory of light. If the light were bright enough it seemed reasonable, given enough time, that an electron in an atom might acquire enough energy to escape regardless of the frequency. The answer was finally provided in 1905 by Albert Einstein who suggested that light, at least sometimes, should be considered to be composed of small bundles of energy or particles called photons. This approach had been used a few years earlier by Max Planck in his successful explanation of black body radiation . In 1907 Einstein was awarded the Nobel Prize in physics for his explanation of the photoelectric effect.


The Einstein photoelectric theory

Einstein's explanation of the photoelectric effect was very simple. He assumed that the kinetic energy of the ejected electron was equal to the energy of the incident photon minus the energy required to remove the electron from the material, which is called the work function. Thus the photon hits a surface, gives nearly all its energy to an electron and the electron is ejected with that energy less whatever energy is required to get it out of the atom and away from the surface. The energy of a photon is given by E = hg = hc/l where g is the frequency of the photon, l is the wavelength, and c is the velocity of light. This applies not only to light but also to x rays and gamma rays. Thus the shorter the wavelength the more energetic the photon.

Many of the properties of light such as interference and diffraction can be explained most naturally by a wave theory while others, like the photoelectric effect, can only be explained by a particle theory. This peculiar fact is often referred to as wave-particle duality and can only be understood using quantum theory which must be used to explain what happens on an atomic scale and which provides a unified description of both processes.


Applications

The photoelectric effect has many practical applications which include the photocell, photoconductive devices and solar cells. A photocell is usually a vacuum tube with two electrodes. One is a photosensitive cathode which emits electrons when exposed to light and the other is an anode which is maintained at a positive voltage with respect to the cathode. Thus when light shines on the cathode, electrons are attracted to the anode and an electron current flows in the tube from cathode to anode. The current can be used to operate a relay, which might turn a motor on to open a door or ring a bell in an alarm system. The system can be made to be responsive to light, as described above, or sensitive to the removal of light as when a beam of light incident on the cathode is interrupted, causing the current to stop. Photocells are also useful as exposure meters for cameras in which case the current in the tube would be measured directly on a sensitive meter.

Closely related to the photoelectric effect is the photoconductive effect which is the increase in electrical conductivity of certain non metallic materials such as cadmium sulfide when exposed to light. This effect can be quite large so that a very small current in a device suddenly becomes quite large when exposed to light. Thus photoconductive devices have many of the same uses as photocells.

Solar cells, usually made from specially prepared silicon, act like a battery when exposed to light. Individual solar cells produce voltages of about 0.6 volts but higher voltages and large currents can be obtained by appropriately connecting many solar cells together. Electricity from solar cells is still quite expensive but they are very useful for providing small amounts of electricity in remote locations where other sources are not available. It is likely however that as the cost of producing solar cells is reduced they will begin to be used to produce large amounts of electricity for commercial use.


Resources

books

Serway, Raymond, Jerry S. Faughn, and Clement J. Moses. College Physics. 6th ed. Pacific Grove, CA: Brooks/Cole, 2002.

periodicals

Chalmers, Bruce. "The Photovoltaic Generation of Electricity." Scientific American 235, no. 4 (October 1976): 34-43.

Stone, Jack L. "Photovoltaics: Unlimited Electrical Energy From the Sun." Physics Today (September 1993): 22-29.

Zweibel, Ken. "Thin-Film Photovoltaic Cells." American Scientist 81 no. 4 (July-August 1993).


Robert Stearns

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Photocell

—A vacuum tube in which electric current will flow when light strikes the photosensitive cathode.

Photoconductivity

—The substantial increase in conductivity acquired by certain materials when exposed to light.

Photoelectric effect

—The ejection of an electron from a material substance by electromagnetic radiation incident on that substance.

Photoelectron

—Name given the electron ejected in the photoelectric effect.

Solar cell

—A device by which sunlight is converted into electricity.

Work function

—The amount of energy required to just remove a photoelectron from a surface. This is different for different materials.

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