Physics
The Photoelectric Effect
The photoelectric effect refers to the phenomenon where electrons are emitted from a material's surface when it is exposed to light. This effect provided evidence for the particle-like behavior of light, as described by Albert Einstein in 1905. The energy of the emitted electrons is directly proportional to the frequency of the incident light, rather than its intensity.
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11 Key excerpts on "The Photoelectric Effect"
eBook - PDF- William Moebs, Samuel J. Ling, Jeff Sanny(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
It was perceived, even by Planck himself, as a useful mathematical trick that led to a good theoretical “fit” to the experimental curve. This perception was changed in 1905 when Einstein published his explanation of The Photoelectric Effect, in which he gave Planck’s energy quantum a new meaning: that of a particle of light. Chapter 6 | Photons and Matter Waves 253 6.2 | Photoelectric Effect Learning Objectives By the end of this section you will be able to: • Describe physical characteristics of The Photoelectric Effect • Explain why The Photoelectric Effect cannot be explained by classical physics • Describe how Einstein’s idea of a particle of radiation explains The Photoelectric Effect When a metal surface is exposed to a monochromatic electromagnetic wave of sufficiently short wavelength (or equivalently, above a threshold frequency), the incident radiation is absorbed and the exposed surface emits electrons. This phenomenon is known as The Photoelectric Effect. Electrons that are emitted in this process are called photoelectrons. The experimental setup to study The Photoelectric Effect is shown schematically in Figure 6.8. The target material serves as the anode, which becomes the emitter of photoelectrons when it is illuminated by monochromatic radiation. We call this electrode the photoelectrode. Photoelectrons are collected at the cathode, which is kept at a lower potential with respect to the anode. The potential difference between the electrodes can be increased or decreased, or its polarity can be reversed. The electrodes are enclosed in an evacuated glass tube so that photoelectrons do not lose their kinetic energy on collisions with air molecules in the space between electrodes. When the target material is not exposed to radiation, no current is registered in this circuit because the circuit is broken (note, there is a gap between the electrodes).
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
Although based upon quantum mechanics, the method treats the incident light as an electro-magnetic wave that causes an atom and its constituent electrons to transition from one energy state (eigenstate) to another. While one can use the classical electromagnetic theory of light to describe the effect, one may also use the modern quantum theory of light to describe The Photoelectric Effect. However, the modern quantum theory of light is not a particle model, as it does not always predict results which one would expect from a naïve particle interpretation. An example would be in the dependence on polarization with regard to the direction electrons are emitted, a phenomenon that has been considered useful in gathering polarization data from black holes and neutron stars. Traditional explanation The photons of a light beam have a characteristic energy determined by the frequency of the light. In the photoemission process, if an electron within some material absorbs the energy of one photon and thus has 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 emitted, but does not increase the energy that each electron possesses. Thus the energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy of the individual photons. (This is true as long as the intensity is low enough for non-linear effects caused by multiphoton absorption or level shifts such as the AC Stark effect to be insignificant. This was a given in the age of Einstein, well before lasers had been invented.) Electrons can absorb energy from photons when irradiated, but they usually follow an all or nothing principle.
eBook - ePub- Jerry Marion(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
17ELECTRONS AND PHOTONS
Publisher Summary
This chapter explains the phenomenon of photoelectric effect. The Photoelectric Effect exhibits a frequency limit, and if the frequency is too low, the effect cannot be produced even if the light intensity is made very large. The chapter describes the qualitative aspects of The Photoelectric Effect. According to classical electromagnetic theory, the energy transferred by a wave is proportional to its intensity and does not depend on the frequency. The chapter discusses the fundamental principles governing the interaction of radiation and matter at the atomic level. It illustrates the interference pattern produced by electrons. Electrons have wave properties and exhibit interference effects. Electron wavelengths tend to be considerably smaller than the wavelengths of visible light. The chapter explains the accumulation of single-photon events on a diffraction screen. A photon can exhibit both wavelike and particlelike properties. The chapter discusses the concept of wave packet and describes quantum theory.At about the time that Einstein was formulating his ideas concerning space and time which were to lead to the development of relativity theory, scientists were also investigating the nature of light and electrons. Electrons were known to be particles and light was acknowledged to be a wave phenomenon.We all have rather clear intuitive ideas about waves and particles. We know that a wave is an extended propagating disturbance in a medium. We know that a particle is an object that can be located at a particular point in space whereas a wave cannot. And we have come to accept the existence of atomic particles–electrons, protons, and neutrons. What could be simpler? A wave is a wave, and a particle is a particle; the distinction is clear.17-1 The Photoelectric Effect
The Ejection of Electrons from Metals
But it is not all this simple. As the 20th century began, scientists were confronted with new questions concerning waves and particles. It had been discovered, for example, that if a piece of clean zinc is exposed to ultraviolet (UV) radiation, the zinc acquires a positive charge. The radiation can carry no charge to the zinc, so this result must mean that electrons (the carriers of negative charge) are literally knocked off the zinc by the action of the UV radiation (Fig. 17-1 ). The removal of electrons causes the zinc to become charged positively. This phenomenon is called the photoelectric effect and the ejected electrons are called photoelectrons.
eBook - PDF- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
Additional examples, video, and practice available at WileyPLUS 38-2 The Photoelectric Effect Learning Objectives After reading this module, you should be able to . . . 38.03 Make a simple and basic sketch of a photoelectric experiment, showing the incident light, the metal plate, the emitted electrons (photoelectrons), and the collector cup. 38.04 Explain the problems physicists had with The Photoelectric Effect prior to Einstein and the historical importance of Einstein’s explanation of the effect. 38.05 Identify a stopping potential V stop and relate it to the maximum kinetic energy K max of escaping photoelectrons. 38.06 For a photoelectric setup, apply the relationships between the frequency and wavelength of the inci- dent light, the maximum kinetic energy Kmax of the photoelectrons, the work function Φ, and the stopping potential V stop . 38.07 For a photoelectric setup, sketch a graph of the stopping potential V stop versus the frequency of the light, identifying the cutoff frequency f 0 and relating the slope to the Planck constant h and the elementary charge e. ● When light of high enough frequency illuminates a metal surface, electrons can gain enough energy to escape the metal by absorbing photons in the illumina- tion, in what is called The Photoelectric Effect. ● The conservation of energy in such an absorption and escape is written as hf = K max + Φ, where hf is the energy of the absorbed photon, K max is the kinetic energy of the most energetic of the escaping electrons, and Φ (called the work function) is the least energy required by an electron to escape the electric forces holding electrons in the metal. ● If hf = Φ, electrons barely escape but have no kinetic energy and the frequency is called the cutoff frequency f 0 . ● If hf < Φ, electrons cannot escape. Key Ideas
eBook - PDF- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Figure 38.2.2 is a plot of V stop versus f . Note that The Photoelectric Effect does not occur if the frequency is below a certain cutoff fre- quency f 0 or, equivalently, if the wavelength is greater than the corresponding cutoff wavelength λ 0 = c/f 0 . This is so no matter how intense the incident light is. This is another puzzle for classical physics. If you view light as an electro- magnetic wave, you must expect that no matter how low the frequency, electrons can always be ejected by light if you supply them with enough energy—that is, if you use a light source that is bright enough. That is not what happens. For light below the cutoff frequency f 0 , The Photoelectric Effect does not occur, no matter how bright the light source. Sliding contact V Vacuum Quartz window Incident light C T i i A + – + – Figure 38.2.1 An apparatus used to study The Photoelectric Effect. The incident light shines on target T, ejecting electrons, which are col- lected by collector cup C. The electrons move in the circuit in a direction opposite the conventional current arrows. The batteries and the variable resistor are used to produce and adjust the electric potential dif- ference between T and C. 1229 38.2 The Photoelectric Effect The existence of a cutoff frequency is, however, just what we should expect if the energy is transferred via photons. The electrons within the target are held there by electric forces. (If they weren’t, they would drip out of the target due to the gravitational force on them.) To just escape from the target, an electron must pick up a certain minimum energy Φ, where Φ is a property of the target material called its work function. If the energy hf transferred to an electron by a photon exceeds the work function of the material (if hf > Φ), the electron can escape the target. If the energy transferred does not exceed the work function (that is, if hf < Φ), the electron cannot escape.
eBook - PDFHigh Resolution Imaging
Detectors and Applications
- Swapan K. Saha(Author)
- 2015(Publication Date)
- Jenny Stanford Publishing(Publisher)
A photoelectric light detection occurs when a photon interacts with a sensor to create an electrical signal. The interaction of light of appropriate frequency with materials results in the absorption of photons and the creation of electron–hole pairs (see Section 2.2.4.1). The photon absorption is accomplished by a change in the energy state of an electron in the sensor material. If the energy is sufficient to release the electrons from the substance, the charge carriers under the influence of an external electric field drift to the electrodes of opposite polarity produce an electric current, called photocurrent. For the light with frequencies below a certain cut-off value, ν c , the photoemission does not occur. For ν > ν c , where ν c is the cut-off frequency of the photon required to produce photoelectric emission and is a constant for the material, a major fraction of the excess energy [ = h ( ν − ν c )] appears as kinetic energy of the emitted electron. The non-measurable time lag between the incidence of the radiation and the ejection of the electron follows from the corpuscular nature of the radiation. When an oscillator emits a photon, it drops from energy, nh ν to a level ( n − 1) h ν . The energy of the photon is expended in liberating the electron from the metal and imparting a velocity to it. The maximum kinetic energy, E k , of the ejected electrons is linearly related to the energy of the absorbed photons, E ( = h ν ), and the work function, φ 0 , of the PE surface (see Table 5.1) E k = h ν − φ 0 = h ν − h ν c = h ( ν − ν c ), (2.1) 82 High Resolution Imaging: Detectors and Applications with E k ( = 1 / 2 mv 2 ) as the kinetic energy of the ejected photoelec-tron from the metal surface, m the mass of the electron, v the velocity of the electron, and ν c the cut-off frequency, which is a characteristic of the metal. The work function of a metal is defined as the minimum energy, measured in electron volts, needed to release an electron from atomic binding.
eBook - PDFLet There Be Light: The Story Of Light From Atoms To Galaxies (2nd Edition)
The Story of Light from Atoms to Galaxies
- Alex Montwill, Ann Breslin(Authors)
- 2013(Publication Date)
- ICP(Publisher)
It varies from metal to metal, and determines the minimum photon energy required for The Photoelectric Effect to take place at that metallic surface. Photoelectric work functions of some elements. Element Al Cs Cu Hg K Na Pb W (eV)* 4.28 2.14 4.65 4.49 2.30 2.75 4.25 The most energetic photons of visible light are at the violet end of the spectrum. It can be seen by comparing the data in the tables above and below that such photons have enough energy to liberate electrons from caesium, potassium and sodium, but not from aluminium, copper, mercury or lead. Energies of some visible photons. Wavelength (nm) 400 550 700 Photon energy, hf (eV) 3.11 2.26 1.61 Practical applications Quite apart from its theoretical importance, The Photoelectric Effect has numerous practical applications. It makes it possible to convert a light signal into an electric current. Television cameras, burglar alarms, barcode readers and light sensors of every descrip-tion are based on The Photoelectric Effect. Perhaps the most remarkable practical example is the photo-multiplier , which amplifies the electrical signal to such an extent that it is possible to detect the arrival of a single photon. The use 392 Let There Be Light 2nd Edition of this instrument in some fascinating fundamental experiments with individual photons will be discussed in Chapter 14. In 1907, the Russian scientist Boris Rosing (1869–1933) realized that light from a cathode ray tube could be thrown onto a screen and made into a pic-ture. One of his students, Vladimir Kosma Zworykin (1889–1982), was fascinated by this notion and applied for a pat-ent for what was in effect the idea for the first television cam-era. The patent application was lodged in 1923, although Zworykin had no working model at the time. Zworykin went to work for Westinghouse Electric and Manufacturing Company in America and began to develop a practical version of the camera tube called the iconoscope.
- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2023(Publication Date)
- Wiley(Publisher)
1164 CHAPTER 38 Photons and Matter Waves An aside: An explanation of The Photoelectric Effect certainly requires quantum physics. For many years, Einstein’s explanation was also a compelling argument for the existence of photons. However, in 1969 an alternative explanation for the effect was found that used quantum physics but did not need the concept of photons. As shown in countless other experiments, light is in fact quantized as photons, but Einstein’s explanation of The Photoelectric Effect is not the best argument for that fact. CHECKPOINT 38.2.1 The figure shows data like those of Fig. 38.2.2 for targets of cesium, potassium, sodium, and lithium. The plots are parallel. (a) Rank the targets according to their work functions, greatest first. (b) Rank the plots according to the value of h they yield, greatest first. V stop Cesium Potassium Lithium Sodium 5.0 5.2 5.4 5.6 5.8 6.0 6.2 f (10 14 Hz) SAMPLE PROBLEM 38.2.1 Photoelectric effect and work function Find the work function Φ of sodium from Fig. 38.2.2. KEY IDEAS We can find the work function Φ from the cutoff fre- quency f 0 (which we can measure on the plot). The rea- soning is this: At the cutoff frequency, the kinetic energy K max in Eq. 38.2.2 is zero. Thus, all the energy hf that is transferred from a photon to an electron goes into the electron’s escape, which requires an energy of Φ. Calculations: From that last idea, Eq. 38.2.2 then gives us, with f = f 0 , hf 0 = 0 + Φ = Φ. In Fig. 38.2.2, the cutoff frequency f 0 is the frequency at which the plotted line intercepts the horizontal fre- quency axis, about 5.5 × 10 14 Hz. We then have Ф = hf 0 = (6.63 × 10 −34 J ⋅ s)(5.5 × 10 14 Hz) = 3.6 × 10 −19 J = 2.3 eV. (Answer) Instructional video is available at the website www.wiley.com After reading this module, you should be able to . . . 38.3.1 For a photon, apply the relationships between momentum, energy, frequency, and wavelength.
eBook - PDF- David Halliday, Robert Resnick, Kenneth S. Krane(Authors)
- 2019(Publication Date)
- Wiley(Publisher)
Figure 45-5, eV 0 K max . V 0 V 45-3 The Photoelectric Effect 1019 Vacuum Quartz window Incident light Electrons E V C i i Sliding contact – + A FIGURE 45-3. An apparatus for studying The Photoelectric Effect. The arrows show the direction of the photoelectric current in the external circuit, which is opposite to the motion of the (neg- atively charged) electrons. The effective potential difference be- tween the emitter E and the collector C, which is read by the volt- meter V, is the algebraic sum of the potential difference associated with the batteries and that associated with the contact potential difference between the emitter and the collector. Because these components are made of different materials they form a “battery” in their own right. FIGURE 45-4. A plot (not to scale) of data taken with the ap- paratus of Fig. 45-3. The intensity of the incident light is twice as great for curve b as for curve a. The emitter and the wavelength of the incident light are the same for both runs. Note that the stop- ping potential is the same for each run. Current i Potential difference ∆V b a V 0 0 – + in which we plot the stopping potential against the fre- quency (rather than the wavelength) of the incident light, shows the result for an emitter made of sodium. The plot is a straight line with an intercept f 0 on the frequency axis, suggesting a second fact about The Photoelectric Effect: Fact 2. The frequency of the light falling on a given emitter must be greater than a certain value f 0 . Other- wise The Photoelectric Effect will not occur. This cutoff frequency f 0 depends only on the material of which the emitter is made and is totally independent of the intensity of the incident light. A third fact about The Photoelectric Effect has been firmly established by separate experiments: Fact 3. Photoelectrons are emitted without delay once the incident light reaches the surface of the emitter.
eBook - ePub- Mario Bertolotti(Author)
- 2004(Publication Date)
- CRC Press(Publisher)
Annalen der Physik in 1902. In this paper he reported two important facts: the first one was that in order to obtain electrons from a given metal surface only light of certain frequencies was effective; the second fact regarded the velocity (kinetic energy) of the emitted electrons that did not depend on the intensity of the incident radiation.Einstein in his work provided an explanation of The Photoelectric Effect as an example of the application of his theory of light quanta. According to him, the energy of the light wave does not propagate as a wave, but rather as a particle (Einstein called it a ‘quantum of energy’) that has an energy inversely proportional to the wavelength of the light. The number of quanta is proportional to the light intensity. The more intense a wave is, the more quanta it contains. When a quantum of light collides with an electron in the metal, it gives the electron all its energy and disappears. The electron spends part of this energy escaping from the metal, and keeps what remains as kinetic energy. The intensity of the light beam, being proportional to the number of quanta, has no effect on the energy of the electrons, but determines their total number.In a letter to his friend Conrad Habicht (1876–1958), Einstein wrote of his paper:‘It deals with radiation and the energy characteristics of light and is very revolutionary, as you will see.’Notwithstanding this declaration, in discussing the physical interpretation of Wien’s law and enouncing the concept of a quantum of light, Einstein did not consider that he had broken with tradition. By introducing the quantum of light he applied in a coherent way the statistical methods associated with the theory of radiating heat. However, he called his introduction of the hypothesis of light quanta a ‘revolutionary’ step because he thought it contradicted Maxwell’s electrodynamics that required that radiation to be a continuously radiating energy flow in space.
- David Halliday, Jearl Walker, Patrick Keleher, Paul Lasky, John Long, Judith Dawes, Julius Orwa, Ajay Mahato, Peter Huf, Warren Stannard, Amanda Edgar, Liam Lyons, Dipesh Bhattarai(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
• The conservation of energy in such an absorption and escape is written as hf = K max + ϕ, where hf is the energy of the absorbed photon, K max is the kinetic energy of the most energetic of the escaping electrons, and Φ (called the work function) is the least energy required by an electron to escape the electric forces holding electrons in the metal. • If hf = Φ, electrons barely escape but have no kinetic energy and the frequency is called the cutoff frequency f 0 . • If hf < Φ, electrons cannot escape. The Photoelectric Effect FIGURE 38.1 An apparatus used to study The Photoelectric Effect. The incident light shines on target T, ejecting electrons, which are collected by collector cup C. The electrons move in the circuit in a direction opposite the conventional current arrows. The batteries and the variable resistor are used to produce and adjust the electric potential difference between T and C. Sliding contact V Vacuum Quartz window Incident light C T i i A + – + – If you direct a beam of light of short enough wavelength onto a clean metal sur- face, the light will cause electrons to leave that surface (the light will eject the electrons from the surface). This photoelectric effect is used in many devices, including camcorders. Einstein’s photon concept can explain it. Let us analyse two basic photoelectric experiments, each using the apparatus of figure 38.1, in which light of frequency f is directed onto target T and ejects electrons from it. A potential difference V is maintained between target T and collector cup C to sweep up these electrons, said to be photoelectrons. This collection produces a photoelectric cur- rent i that is measured with meter A. First photoelectric experiment We adjust the potential difference V by mov- ing the sliding contact in figure 38.1 so that collector C is slightly negative with respect to target T. This potential difference acts to slow down the ejected electrons.
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