The Photoelectric Effect

The Photo galvanizing imprintAssessment project Topic The photoelectricalalal Effect1. IntroductionThe photoelectric way out is the remark given to the phenomenon whereby electrons are emitted from a coat when exposed to electromagnetic irradiation therapy of the appropriate relative frequence. It was first disc overed by Heinrich Hertz in 1887, still remained a paradox to many scientists who sought to explain it, as it clear contradicted the accepted principles of uncorrupted natural philosophy such as James salesclerk Maxwells Theory of Electromagnetic Waves. This phenomenon, unable to be explained by the cockle dumbfound of weightlessness, was finally explained by Albert whiz in 1905 with the inception of his Quantum Theory, a notion that would on the whole revolutionise scientific thought. The photoelectric meat has solveed and continues to play an important role in mankinds scientific development.2. Discovery of the photoelectrical Effect HertzThe origin al observation of the photoelectric resultant toilet be traced back to the German scientist Heinrich Hertz. In 1887, in an attempt to generate and pick up electromagnetic radiation, Hertz created a rapidly-oscillating electric field with a amply potency k straightwayledgeability coil to cause a sack evacuate between two spherical brass electrodes. He detect that when a atomic length of copper wire with brass spheres attached on either end was bent into a grummet, loss a downhearted gap between the spheres, and held near the sparking foundation coil, a spark would jumpstart across the gap at the same time when the brass electrodes in the induction loop sparked. This induced spark occurred despite the copper loop not being connected to any electrical current source. in that respectfromly Hertz came to the conclusion that the copper loop was a detector of the electromagnetic jars propagated by the transmitting loop.This successful experiment was followed up by a ser ies of others, through which Hertz demonstrated that these electromagnetic waves could be reflected from a coat mirror, and refracted as they passed through a prism do from pitch, therefore proving that these waves behaved similarly to sluttish waves. He overly proved these waves were polarised.Through the flux of his investigations, he discovered a mysterious phenomenon I occasionally wrap the spark Bthe detector sparkin a dark case so as to more easily dedicate the observations and in so doing I notice that the uttermost spark-length became decidedly smaller in the case than it was before. On removing in succession the various parts of the case, it was seen that the whole portion of it which exercised this prejudicial set was that which screened the spark B from the spark Athe transmitter spark. The partition on that expression exhibited this effect, not exclusively when it was in the immediate neighbourhood of the spark B, nevertheless also when it was interposed a t peachyer distances from B between A and B. A phenomenon so remarkable called for closer investigation.Upon shielding the detecting loop with glass, the excitement of the spark produced was reduced. However, when a quartz shield (a substance that allows UV barbs to pass) was applied, there was no drop in the spark forcefulness level. He thence apply a quartz prism to separate the swinging from the transmitter spark into its various components, discovering that the wavelength which made the detector spark more powerful was in the ultraviolet range. Unable to explain this phenomenon, Hertz concluded his series of investigations in 1887, declaring that I confine myself at present to communicating the results obtained, without attempting any possibility respecting the manner in which the observed phenomena are brought about.3. Further Investigations Hallwachs, Thomson, von LenardAfter encyclopedism of Hertzs experiments, another German scientist, Wilhelm Hallwachs, devised a m uch simpler investigation to demonstrate the photoelectric effect. In his own words In a recent issuing Hertz has described investigations on the dependence of the maximum length of an induction spark on the radiation received by it from another induction spark. He proved that the phenomenon observed is an activity of the ultraviolet easy. No merely set about on the nature of the phenomenon could be obtained, because of the complicated conditions of the rese sackh in which it appeared.I have endeavored to obtain related phenomena which would occur under simpler conditions, in order to make the explanation of the phenomena easier. Success was obtained by investigating the action of the electric light on electrically pullulated bodies. By placing a zinc plate atop an insulating stand and wiring it to a negatively-charged notes leaf electroscope, he observed a slow loss of charge from the electroscope. However, when he exposed the zinc plate to ultraviolet light from an arc lamp or from burning magnesium, the discharge occurred much quicker. Conversely, a positively-charged electroscope resulted in no fast leakage of charge.In 1899, British scientist J.J. Thomson finally identified that the light cause the surface go forth to emit electrons. He enclosed the metal in an evacuated tube before exposing it to radiation, showing the electrons to be the same particles emitted in cathode radiation therapy tubes.Three years later, German physicist Philipp von Lenard, who had usageed with Hertz earlier in Bonn, conducted a series of experiments in which he used a bright vitamin C arc light to examine how the efficacy of the emitted electrons wide-ranging with the lights intensity (see send off 2). By using a vacuum tube, he showed that when electrons emitted by the metal plate upon exposure to light hit another plate, the collector, a small measurable current was produced. By charging the collector negatively so as to repel the electrons, von Lenard discover ed that a minimum finage existed, V eat up, so that only electrons with a genuine ability threshold could reach the collector and thus generate a current.He found that while increase light intensity caused more electrons to be emitted (as can be gathered from an observed increase in current), it did not affect the amount of thrust carried by each electron, as the fish filet voltage was constant. On the other hand, increasing the oftenness of the light led to an augmentation in the electrons energising cipher, thus purpose that for a particular frequency of light, the energizing push button of the electrons remained constant. Von Lenard also showed that if the frequency was lowered beyond a certain threshold, no current was produced, disregardless of the intensity of the light. However, like the scientists preceding him, he was unable to military issue for these phenomena.4. deficiency of Classical Physics business relationshipsThe phenomenon observed during the photo electric effect was in contradiction to classical system explanations such as Maxwells Theory of Electromagnetic Waves which was then commonly accepted by scientists. According to such rules of classical physics, for an electron to gain enough goose egg to be emancipate from the metal, the metal fold would have to be exposed to the light waves for a period of time. However, as observed in experiments of the photoelectric effect, the electrons were freed instantly. The Wave Theory maintains that increasing the intensity of a beam of light also increases the amplitude of the oscillating electric field vector E, thus the amount of electrons emitted should be proportional to the intensity of the light.However, according to the observations made, the current flow was in low-level of light intensity, yet varied according to the frequency of the light, and was non-existent when the frequency decreased beyond a certain level, regardless of the intensity. Von Lenards experiment confirme d the existence of a threshold frequency in the photoelectric effect, another phenomenon unable to be explained with a classical physics approach. therefore the belief in light being completely wavelike in nature was incompatible with the observational observations of the photoelectric effect.5. mordant Body beam and Plancks HypothesisA portentous luggage compartment stone can be defined as a perfect pit that absorbs all radiation that falls onto it and then perfectly radiates all energy absorbed until it is at equilibrium with its surroundings. The intensity of various wavelengths emitted by the black body changes according to its temperature, forming black body radiation curves (see diagram on right). Experimental data showed that the intensity of radiation emitted increased with decreasing wavelength, until a definite peak is reached, afterwards which lower wavelengths of radiation are emitted at lower intensities.Yet, according to the classical wave theory of light, as the wavelength of the radiation emitted shortened, the intensity should increase, thus as the wavelength tends to zero, intensity would approach infinity. However, this would be a gross violation of the principle of conservation of energy. Hence it remained an inexplicable conundrum for scientists for a long time, who gave this effect the name ultraviolet catastrophe.In 1900, German scientist Max Planck came up with a revolutionary explanation for this phenomenon. He made the hypothesis that the radiant energy may be treated statistically not as continuous waves but rather as distinct packets of energy, each of which he called a quantum. Based on this radical assumption of light as particles, he formulated a mathematical comparability by which this phenomenon could be exemplified. He proposed this relation that calculated the energy of a quantum for radiation of a certain frequency E= hf,Ebeing the energy in joules, fthe frequency in Hertz, and ha small constant (6.626 x 10-34Js) now known as Plancks constant. Figure 4 is a graph of experimental results that confirms Plancks equation, with the gradient corresponding to h. He proposed that any quanta of a particular frequency (and thus wavelength) would carry the same amount of energy. However, he did not dimension any physical significance to this postulation, merely perceiving it as a mathematical trick by which the corresponding answer could be obtained.6. Quantum Theory mavins ExplanationDue to the inadequacies of classical physics in explaining the photoelectric effect, in 1905 Albert Einstein further developed upon Plancks hypothesis to come up with a unexampled ground-breaking theory to explain the photoelectric effect. He proposed that light was made up not of continuous waves but rather of discrete bundles of energy which he termed photons. He wrote in the renowned journal Annalen der Physik It seems to me that the observations on black-body radiation, photoluminescence, the payoff of cathode ra ys by ultraviolet light and other phenomena involving the emission or conversion of light can be better unders tood on the assumption that the energy of light is distributed discontinuously in space.According to the assumption considered here, when a light ray starting from a point is propagated, the energy is not continuously distributed over an ever increasing volume, but it consists of a finite number of energy quanta, localised in space, which move without being divided and which can be absorbed or emitted only as a whole.Einstein used Plancks equation that each photon had an energy E=hf, and proposed that light intensity was proportional to the number of photons. The higher the frequency of the electromagnetic radiation, the greater the energy carried by its photons. Einstein provided a comprehensive explanation for the photoelectric effect. When an electron is resignd from the metal surface, the energy in the light photons must be great enough to overcome the forces that bind the electrons to the surface. This minimum energy required to liberate an electron from a metal surface is known as the add function, represented by the symbol , and is dependent solely on the sensible of the metal.The corresponding minimum frequency required for the photons to contain the required energy is called the threshold frequency (f0). If the energy of the photon is greater than the work function of the metal (i.e. E hf0), than the difference in their energy levels will provide the energising energy for the photoelectrons (electrons released from interaction with a photon), allowing them to travel and thus generate an electric current. Einsteins quantum theory explains the existence of a threshold frequency for the light below which no electrons would be emitted from the metal, an experimental observation that had puzzled scientists up to that time.Einstein formal that when different metal surfaces are illuminated with monochromatic light, photoelectrons are emitted by the metal surface. The magnitude of the forces by which electrons are held varies with different metals. Thus the work functions of each different metal are also varied. Below is a table of the work functions of various metals.Figure 6Work Functions for versatile MetalsSource Nave, CR. HyperPhysics Photoelectric EffectAccording to Einsteins theory a ace photon collides with an electron in the metal, transferring all its energy to the electron, thus liberating the (photoelectron from the metal surface. This archetype successfully explained the instantaneity of the electron emission upon light exposure, another phenomenon that classical wave theory was unable to account for.In Einsteins own words, According to the supposition that the incident light consists of energy quanta one can picture the production of cathode rays by light as follows. Energy quanta penetrate into a surface layer of the body, and their energy is at least partly transformed into electron kinetic energy. The simplest picture is that a light quantum transfers all of its energy to a single electron we shall assume that that happens. We must, however, not exclude the possibility that electrons only receive part of the energy from light quanta.An electron obtaining kinetic energy inside the body will have lost part of its kinetic energy when it has reached the surface. Moreover, we must assume that each electron on leaving the body must produce work P, which is characteristic for the body. Electrons which are horny at the surface and at right angles to it will leave the body with the greatest normal velocity.Einstein formulated an equation, known as Einsteins Photoelectric Equation, to provide a quantitative explanation for the photoelectric effectE= hf= + Ekwith Ebeing the energy of the photon (thus E= hffrom Plancks hypothesis),the work function of the particular metal (= hf0), and Ekthe photoelectrons kinetic energy (in Joules or electron volts).Einsteins theory also explains the stoppi ng voltage in the photoelectric effect, which von Lenard had discovered earlier. This voltage is a unattackable measure of the kinetic energy of the photoelectrons. It can be demonstrated (see excogitation 7) by introducing a variable electric potential difference to make the anode negative, thus generating a repelling force against the photoelectrons emitted from the cathode. As this opposing voltage is increased, it will arrive at a point where there is no current flowing in the out-of-door circuit as the photoelectrons kinetic energy is not enough to overcome the voltage. This stopping potential equals the maximum kinetic energy of the electrons at the cathode, as it is just enough to stop any electron from reaching the anode.Thus EK max= -qV0, where EK maxis the maximum kinetic energy of the electron in joules, V0the magnitude of the stopping potential in volts, and q the charge of the electron (-1.60 x 10-19C). As the unit of the joule is too large to be used effectively for atomic systems, the electron volt (eV) is employed instead, with 1 eV = 1.60 x 10-19J. Thus the maximum kinetic energy of a photoelectron can be experimentally obtained from the stopping voltage. Radiation with higher frequencies will result in higher stopping voltages, and wrong-doing versa.With his theory of the quantisation of light, Einstein was able to derive Plancks formula and account directly for such hitherto inexplicable phenomena as the photoelectric effect and black-body radiation. His work overturned the previously accepted, but now proven flawed, wave theory of light, heralding a new era with the concept of wave-particle duality, in which light can be seen both as waves and as particles (quanta). It was for his services to hypothetical Physics, and especially for his discovery of the law of the photoelectric effect that Einstein was awarded the Nobel hold dear for Physics in 1921.Another notable scientist, the American Robert Millikan, expressed scratch up doubts about Einsteins quantum theory and set out to experimentally prove him wrong. However, after a decade of thorough scientific investigations, Millikans results confirmed Einsteins theory in every aspect. He was even able to measure Plancks constant to at bottom 0.5% accuracy. These travails earned Millikan the Nobel Prize in 1923 and further validated Einsteins quantum theory in explaining the photoelectric effect.7. Practical Applications of the Photoelectric EffectThe principle of the photoelectric effect is utilised in many domains. One significant application of the photoelectric effect is the solar cell. This is a device that converts electromagnetic radiation from cheerfulness into electrical energy. It is generally made up of a series of metal(prenominal) plates facing the sun, emitting photoelectrons when struck by sunlight. These electrons then flow through an external circuit, thus generating electrical power.Another practical application is the photomultiplier tube (PMT ). When light is shone onto a photosensitive cathode, electrons are emitted, and subsequently accelerated towards a second base cathode. This produces more electrons, and is repeated for a number of cathodes, resulting in the multiplication of the number of electrons initially emitted by a factor of a million, to be sight as a current pulse at the final electrode. Thus PMTs are extremely sensitive light detectors, used in scientific applications that require high levels of accuracy, such as emission spectroscopy experiments.Phototubes also operate on the principle of the photoelectric effect. The electrical characteristics of these devices are dependent on the light that they are exposed to. Thus the current produced from a phototube may be used to operate sensor-based appliances such as machine rifle doors, sensor taps, alarm systems and light-activated counters.8. ConclusionThe photoelectric effect has undeniably vie a significant role in the development of modern physics ever since its discovery. It has revolutionised mankinds understanding of the nature of light, its wave-particle duality. It was in the pursuit of an explanation for this phenomenon that Einstein made what was an important great leap forward in the arena of science 3/4his conception of quantum theory. In fact the photoelectric effect and the problem of the ultraviolet catastrophe in black-body radiation formed the two experimental foundations upon which quantum theory was built.Thus the experiments conducted on the photoelectric effect can be considered among the most significant in the history of physics. Three peremptory physicists received the Nobel Prize in part for their work on the photoelectric effect Max Planck in 1918, Albert Einstein in 1921 and Robert Millikan in 1923. The observations of the photoelectric effect and its subsequent explanations by Einstein can be regarded as directly responsible for the birth of modern physics.AppendixA Timeline of the Photoelectric EffectB ibliographyAndriessen, M et al. Physics 2 HSC Course2nded. Sydney John Wiley Sons Australia 2003.Burns, RW. 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