Modern Physics
“Photoelectric Effect”
Arranged By:
Ignasius Bagus Asmarianto
Megawaty Elyna Magnolia Kumambong
Rocky Christofel Mose
Tonny Christian Lampus
Department of Physics
Faculty of Mathematics and Natural Science
Manado State University
2011
PREFACE
The first we thank to God, because only by His blessings we can finish this paper well.
This paper is an assignment of Modern Physics Subject by DR. R. Wenas, Msi. This paper discusses about “Photoelectric Effect”.
We say thanks a lot for everyone who helped us so that this paper has written well.
We hope, this paper can be useful for everyone who read this paper.
The Writers
Table of Content
Preface i
Table Of Content ii
Introductory Understanding 1
Discussion 3
Definition of Photoelectric Effect 3 Characteristic of the Photoelectric Effect 3 Emission Mechanism 5 Experimental Results of the Photoelectric Emission 6 Mathematical Description 7 Experiment of Einstein 7 Stopping Potential 10 Application of Photoelectric Effec 11Conclusion 14
Bibliography 15
The Introductory Understanding
What is the Photoelectric Effect?
Photoelectric Effect is a theory that proposed by Albert Einstein in 1905. Before him, there is some scientist already exposed this concept or theory about Photoelectric Effect. But Einstein is the scientist who solved this problem about Photoelectric Effect.
Albert Einstein solved this apparent paradox by describing light as composed of discrete quanta, now called photons, rather than continuous waves. Based upon Max Planck's theory of black-body radiation, Einstein theorized that the energy in each quantum of light was equal to the frequency multiplied by a constant, later called Planck's constant. A photon above a threshold frequency has the required energy to eject a single electron, creating the observed effect. This discovery led to the quantum revolution in physics and earned Einstein the Nobel Prize in Physics in 1921. By wave-particle duality the effect can be analyzed purely in terms of waves though not as conveniently.
Albert Einstein's mathematical description of how the photoelectric effect was caused by absorption of quanta of light (now called photons), was in one of his 1905 papers, named "On a Heuristic Viewpoint Concerning the Production and Transformation of Light". This paper proposed the simple description of "light quanta", or photons, and showed how they explained such phenomena as the photoelectric effect. His simple explanation in terms of absorption of discrete quanta of light explained the features of the phenomenon and the characteristic frequency. Einstein's explanation of the Photoelectric Effect won him the Nobel Prize in Physics in 1921.
In general definition, Photoelectric Effect is change process of electric conduction characteristics in material caused by light or other electromagnetic wave. This effect makes electron pair and hole in semiconductor, or free electron radiation and ion in metal. The first phenomenon is Internal Photoelectric Effect, and the second phenomenon is External Photoelectric Effect.
Photoelectric Effect |
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Light-Matter Interaction |
In the Photoelectric Effect, electrons are emitted from matter (metals and non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as "photoelectrons". First observed by Heinrich Hertz in 1887, the phenomenon is also known as the "Hertz Effect”, although the latter term has fallen out of general use. Hertz observed and then showed that electrodes illuminated with ultraviolet light create electric sparks more easily.
The photoelectric effect requires photons with energies from a few electron volts to over 1 MeV in high atomic number elements. Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality. Other phenomena where light affects the movement of electric charges include the photoconductive effect (also known as photoconductivity or photo resistivity), the photovoltaic effect, and the photo electrochemical effect.
Light Energy = Threshold Energy + Maximum Kinetic Energy Of Electron
In 1905 Albert Einstein postulated that the electron or particle can receive energy of electromagnetic waves (in the form of light or photons only in the form of discrete (quanta) of
The experiment was perform as follow :
For the given frequency of incident radiation, the stopping potential is independent of its intensity. 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 m is the mass and vmax is the maximum velocity of photoelectron emitted, then
The energy of a photon of all types of electromagnetic radiation is given by the Einstein relation:
Where E: Electric Field
Modern/Quantum Understanding Of The Photoelectric Effect
Discussion
Photoelectric Effect
The photoelectric effect was observed by the following procedure. Two pieces of metal plate (a thin metal plate) that separate placed inside vacuum tubes.
Outside the tube both of plates is connected to each other with wire. At first there was no current flow because the two plates are apart. When the light comes to one plate, electric current is detected on the wire. This occurs as a result of the loose electrons from one plate to other plates together to produce an electric current.
v Characteristic of the Photoelectric Effect
1. Only the appropriate light (which has a frequency greater than a threshold frequency only) that allows the release of electrons from a metal plate or causing the photoelectric effect happens (which is characterized by detection of electric current in the wire). Certain frequencies of light where the electrons released from the metal surface is called the threshold frequency of the metal. This frequency is different for each metal and is a characteristic of the metal.
2. When the light used to produce the photoelectric effect, the addition of light intensity is also coupled with the number of electrons released from a metal plate (which is characterized by the electric current increase). However, the photoelectric effect does not occur for light with a frequency that is smaller than the threshold frequency, although the intensity of light is increased.
3. When photoelectric effect occurs, electrical current was detected in the circuit wire as soon as a light irradiated on the metal plate. This means that almost no interval electrons freed from the metal surface after the metal irradiated by a light.
Characteristics of the photoelectric effect can not be explained using the light wave theory. It takes a new perspective in describing the light where the light was not viewed as a wave that can have a continuous energy but light as a particle.
Theory that describe light as a wave rather than available through discrete or quantized energy concept developed by Planck and proved suitable to describe the radiation spectrum of black body heat. The concept of quantized energy that is used by Einstein to explain the photoelectric effect. Here, the light is viewed as quantum energy that only have a discrete rather than continuous energy that expressed as:
E = hf |
v Emission Mechanism
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 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 electron.
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.
v Experimental Results of the Photoelectric Emission
- For a given metal and frequency of incident radiation, the rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light.
- For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons can be emitted. This frequency is called the threshold frequency.
- For a given metal of particular work function, increase in intensity of incident beam increases the magnitude of the photoelectric current, though stoppage voltage remains the same.
- For a given metal of particular work function, increase in frequency of incident beam increases the maximum kinetic energy with which the photoelectrons are emitted, but the photoelectric current remains the same, though stoppage voltage increases.
- Above the threshold frequency, the maximum kinetic energy of the emitted photoelectron depends on the frequency of the incident light, but is independent of the intensity of the incident light so long as the latter is not too high.
- The time lag between the incidence of radiation and the emission of a photoelectron is very small, less than 10−9 second.
The direction of distribution of emitted electrons peaks in the direction of polarization (the direction of the electric field) of the incident light, if it is linearly polarized.
v Mathematical Description
Where h is the Planck constant and f is the frequency of the incident photon. The term φ = hf0 is the work function (sometimes denoted W), which gives the minimum energy required to remove a delocalized electron from the surface of the metal. The work function satisfies
Where f0 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 f > f0 for the photoelectric effect to occur.
v Experiment of Einstein
Einstein proposed an important concept as the background of the photoelectric effect is that an electron absorbs a quantum of energy. One quantum of energy absorbed is used to release electrons from metals and to move to the other plate. This can be written as:
E = W0 + Ekm hf = hf0 + Ekm Ekm = hf – hf0 |
This equation is called Equation of Einstein's Photoelectric Effect. Note that W0 is the threshold energy of metal or metal work function, f0 is the threshold frequency of the metal, f is the light frequency that used, and Ekm is the maximum kinetic energy of electrons out of metal and moving to another metal plate. In another form of the photoelectric effect equation can be written as :
hf = hf0 + Ekm Ekm = hf – hf0 |
Where m is the electron mass and ve is the speed of electron.Unit of energy in International Standard is Joule (J) and the frequency is hertz (Hz). But, the metal work function is usually expressed in units of electron volts (eV) so keep in mind that
1 eV = 1,602 × 10−19 J hf = hf0 + Ekm Ekm = hf – hf0 |
hf = hf0 + Ekm Ekm = hf – hf0 |
Where and is the frequency of photons light.
The experiment was perform as follow : The experiment was done by Albert Einstein, where he uses light to irradiate Metals Sodium (Sodium) and make observations on the scattered electron at a definite speed.
Scattered electron has kinetic energy ½ mv2 where is the electron mass, and is the speed of the scattered electrons. The movement and acceleration on the electron is a particle property. However, only light and certain photons of light that can scatter electrons in natrium metal surface, namely the photon energy must equal the energy needed to move the electrons (metal work function) add the kinetic energy of the scattered electrons.
hf = hf0 + Ekm Ekm = hf – hf0 |
Where is the minimum energy required to move the electrons are bound to the metal.
Einstein's work predicted that the energy of individual ejected electrons increases linearly with the frequency of the light. It was known that the energy of photoelectrons increases with increasing frequency of incident light and is independent of the intensity of the light. However, the manner of the increase was not experimentally determined until 1915 when Robert Andrews Millikan showed that Einstein's prediction was correct.
v Stopping Potential
Electron movement is characterized as the electric current in the photoelectric effect phenomenon can be stopped by an electrical voltage is mounted on the circuit. If the series voltage source mounted photoelectric effect with reversed polarity (positive pole plate where the source connected with the release of electrons and negative poles of a source is connected to another plate), there is a voltage value that may cause an electric current in the circuit becomes zero.
Zero current or no current means no more electrons are come out from the metal surface due to the photoelectric effect. Voltage values that cause electrons to stop regardless of the metal surface on the photoelectric effect is called voltage or potential or stopping potential.
For the given frequency of incident radiation, the stopping potential is independent of its intensity. 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 m is the mass and vmax is the maximum velocity of photoelectron emitted, thenIf e is the charge on the electron and V0 is the stopping potential, then the work done by the retarding potential in stopping the electron = eV0, which give
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.
This equation is essentially the Energy Equation. Please note that e is the magnitude of electron charge 1.6 × 10-19 C and a voltage is measured in volts (V) unit.
v Application of Photoelectric Effect
1. LED Light (Light Emitting Device)
The photoelectric effect is the basic principle of various photonic devices such as LED lights (light emitting device) and light detector device (photo detector).
2. Dubbing Film
Very surprising if we hear that the first application of photoelectric effect is in the entertainment world. With the help of electronics equipment when it recorded the sound dubbing the film in the form of optical signals along the edge pieces of the film. At the time the movie is playing, this signal is read back through the process of the photoelectric effect and the electrical signal amplified by using a tube amplifier voiced to produce the film.
3. Photomultiplier Tube
The most popular applications in academic circles is photo-multiplier tube. By using this tube almost all the spectrum of electromagnetic radiation can be observed. This tube has a very high efficiency; even it was able to detect even single photons. By using this tube, a group of researchers in Japan Superkamiokande successfully investigates neutrino masses are finally awarded the Nobel Prize in 2002.
4. Photoelectron Spectroscopy or PES
In addition, the external photoelectric effect can also be used for the purposes of spectroscopy through equipment called photoelectron spectroscopy or PES.
Photoelectron spectroscopy utilizes photo-ionization and analysis of the kinetic energy distribution of the emitted photoelectrons to study the composition and electronic state of the surface region of a sample.
Photoelectron spectroscopy is based upon a single photon in/electron out process and from many viewpoints this underlying process is a much simpler phenomenon than the Auger process.
Where:
Photoelectron spectroscopy uses monochromatic sources of radiation (i.e. photons of fixed energy).
5. Photo-Diode or Photo-Transistors
Internal photoelectric effect has more applications that touch the people community. Take for example a photo-diode or photo-transistors that are useful as high-speed light sensor. In fact, in communication fiber optic transmission of 40 Gigabit per second which is equivalent to light pulses along 10 picoseconds (10-11 seconds) still be read by a photo-diode.
6. Solar Cell
Solar cells that we know very beneficial to convert solar energy into electricity through an internal photoelectric effect. A semiconductor is irradiated with visible light will separate the electrons and holes. Excess electrons on one side of which is accompanied by excess hole on the other side will cause a potential difference which, if delivered to the load will produce an electrical current.
7. Camera of Mobile Phones/Digital Camera
Nowadays we are flooded with electronic products that are equipped with a CCD camera (charge coupled device). Call it the camera on mobile phones; digital cameras with resolution up to 12 Megapixels.
8. Barcode Scanner (Image Sensor)
Barcode scanner are used in the supermarkets, all of which utilize an internal photoelectric effect in changing the desired image into electronic data which can then be processed by computer (the screen charged by the photoelectric effect to transform an optical image into a scanned electronic signal).
Conclusion
Photoelectric effect occurs when a beam of light come on the metal, in the event there are electrons that come out of the metal surface.
Classical Understanding about the Photoelectric Effect
a. Electron vibrate (with forced vibration) with amplitude which depends on the intensity. Where the intensity is :
Where E: Electric Field
With a high enough intensity, it makes the electrons can escape although have a low frequency (low ν)
b. According to classical theory, for electrons to escape from the irradiated metal, the electrons take time to expend energy (not soon events).
Modern/Quantum Understanding Of The Photoelectric Effect
a. Frequency must be greater than the threshold frequency ν > ν0 (threshold frequency of the material)
b. Electrons come out with a kinetic energy
c. Soon Events
The photoelectric effect helped propel the then-emerging concept of the dualistic nature of light, that light simultaneously possesses the characteristics of both waves and particles, each being manifested according to the circumstances. The effect was impossible to understand in terms of the classical 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 would 'gather up' energy over a period of time, and then be emitted.
Bibliography
1. ^ a b Serway, Raymond A. (1990). Physics for Scientists & Engineers (3rd ed.). Saunders. pp. 1150. ISBN 0030302587. http://books.google.com/?id=RUMBw3hR7aoC&q=inauthor:serway+photoelectric&dq=inauthor:serway+photoelectric.
2. Krane, Kanneth.1992.Fisika Modern.Jakarta : Penerbit Universitas Indonesia (UI-Prress)
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