Physics
Light Particles
Light particles, also known as photons, are the fundamental particles that make up light. They have properties of both particles and waves, and their behavior is described by quantum mechanics. Photons have no mass and travel at the speed of light, carrying energy and momentum. They play a crucial role in the field of quantum physics and are essential for understanding the nature of light.
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10 Key excerpts on "Light Particles"
eBook - PDF- Frank L. Pedrotti, Leno M. Pedrotti, Leno S. Pedrotti(Authors)
- 2017(Publication Date)
- Cambridge University Press(Publisher)
The final result is that light and subatomic particles, like electrons, are both considered to be mani-festations of energy and are governed by the same set of formal principles. In this introductory chapter, we begin with a brief history of light, addressing it alternately as particle and wave. Along the way we meet the great minds that championed one viewpoint or the other. We follow this account with several basic relationships—borrowed from quantum physics and the special theory of relativity—that describe the properties of subatomic particles, like elec-trons, and the photon . We close this chapter with an introductory glance at the electromagnetic spectrum and a survey of the radiometric units we use to de-scribe the properties of electromagnetic radiation. 1-1 A BRIEF HISTORY 2 In the seventeenth century the most prominent advocate of a particle theory of light was Isaac Newton, the same creative giant who had erected a com-plete science of mechanics and gravity. In his treatise Optics , Newton clearly regarded rays of light as streams of very small particles emitted from a source of light and traveling in straight lines. Although Newton often argued force-fully for positing hypotheses that were derived only from observation and ex-periment, here he himself adopted a particle hypothesis, believing it to be adequately justified by his experience. Important in his considerations was the observation that light seemed to cast sharp shadows of objects, in contrast to water and sound waves, which bend around obstacles in their paths. At the same time, Newton was aware of the phenomenon now referred to as Newton’s rings . Such light patterns are not easily explained by viewing light as a stream of particles traveling in straight lines.
eBook - PDFQuips, Quotes and Quanta
An Anecdotal History of Physics
- Anton Z Capri(Author)
- 2007(Publication Date)
- WSPC(Publisher)
Chapter 8 Particles are Waves are Particles “Are not the rays of Light very small Bodies emitted from shining Sub-stances?” Isaac Newton, Philosophical transactions (1672). At the beginning of the twentieth century, physicists again learned to view optical theory in an earlier light. In the seventeenth century Newton asserted that light was corpuscular or particle-like in nature. A century and a half later the work of luminaries like Thomas Young (1773 – 1829) and Augustin Jean Fresnel (1788 – 1827) definitely showed that, contrary to Newton’s view, light was wave-like and not corpuscular, or particle-like, in nature. By passing light through a slit they demonstrated that light was diffracted just like a sound wave or water wave. It was only the very short wavelengths of visible light that had made it appear to travel like a particle in a straight line without spreading. This viewpoint was about to be reversed. In 1887, at the technological institute at Karlsruhe, Heinrich Rudolf Hertz, only thirty years old, a master of Homer and Greek tragedies, as well as economics and the history of mathematics and physics, was about to open the doors to wireless communication and radio. During the four short years between 1885 and 1889, he not only demonstrated that accelerating charges produce radiation that travels at the speed of light but that this radiation conforms in every respect to the radiation predicted by the theory of electromagnetism produced by James Clerk Maxwell twenty years earlier. After he produced the first electromagnetic (radio) waves, his students were 92 Particles are Waves are Particles 93 impressed and asked him, “What next?” he simply replied, “It’s of no use whatsoever. This is just an experiment that proves Maestro Maxwell was right — we just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there.” Hertz was born in Hamburg where his father was a prominent lawyer and later senator.
eBook - PDFQuips, Quotes and Quanta
An Anecdotal History of Physics
- Anton Z Capri(Author)
- 2011(Publication Date)
- WSPC(Publisher)
Chapter 9 Particles are Waves are Particles “Are not the rays of Light very small Bodies emitted from shining Sub-stances?” Isaac Newton, Philosophical transactions (1672). At the beginning of the twentieth century, physicists again learned to view optical theory in an earlier light. In the seventeenth century Newton asserted that light was corpuscular or particle-like in nature. A century and a half later the work of luminaries like Thomas Young (1773 – 1829) and Augustin Jean Fresnel (1788 – 1827) definitely showed that, contrary to Newton’s view, light was wave-like and not corpuscular, or particle-like, in nature. By passing light through a slit they demonstrated that light was diffracted just like a sound wave or water wave. It was only the very short wavelengths of visible light that had made it appear to travel like a particle in a straight line without spreading. This viewpoint was about to be reversed. In 1887, at the technological institute at Karlsruhe, Heinrich Rudolf Hertz, only thirty years old, a master of Homer and Greek tragedies, as well as economics and the history of mathematics and physics, was about to open the doors to wireless communication and radio. During the four short years between 1885 and 1889, he not only demonstrated that accelerating charges produce radiation that travels at the speed of light but that this radiation conforms in every respect to the radiation predicted by the theory of electromagnetism produced by James Clerk Maxwell twenty years earlier. After he produced the first electromagnetic (radio) waves, his students were impressed and asked him, “What next?” he simply replied, “It’s of no use whatsoever. This is just an experiment that proves Maestro Maxwell was right—we just have these mysterious electromagnetic waves that we cannot 107 108 Quips, Quotes, and Quanta: An Anecdotal History of Physics see with the naked eye.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
The total energy of a (nonrelativistic) particle is the sum of its kinetic energy (KE) and potential energy (PE), or E 5 KE 1 PE. The magnitude p of the particle’s momentum is the product of its mass m and speed v, or p 5 mv. We will see that the ideas of energy and momentum also apply to photons. The defining equations for photon energy and photon momentum, however, are not the same as they are for particles such as electrons and protons. Experimental evidence that light consists of photons comes from a phenomenon called the photoelectric effect, in which electrons are emitted from a metal surface when light shines on it. Figure 29.4 illustrates the effect. The electrons are emitted if the light being used has a sufficiently high frequency. The ejected electrons move toward a positive electrode called the collector and cause a current to register on the ammeter. Because the electrons are ejected with the aid of light, they are called photoelectrons. As will be discussed shortly, a number of features of the photoelectric effect could not be explained solely with the ideas of classical physics. In 1905 Einstein presented an explanation of the photoelectric effect that took ad- vantage of Planck’s work concerning blackbody radiation. It was primarily for his theory of the photoelectric effect that he was awarded the Nobel Prize in physics in 1921. In his photoelectric theory, Einstein proposed that light of frequency f could be regarded as a collection of discrete packets of energy (photons), each packet containing an amount of energy E given by Energy of a photon E 5 hf (29.2) where h is Planck’s constant. The light energy given off by a light bulb, for instance, is carried by photons. The brighter the bulb, the greater is the number of photons emitted per second. Example 1 estimates the number of photons emitted per second by a typical light bulb.
- Paul S Braterman(Author)
- 2012(Publication Date)
- World Scientific(Publisher)
You would be wrong. Each separate quantum of light has acted as a particle, at one specific point on the photographic plate. But the probability of it doing so at any point depends on the intensity of a wave. Did this or that particular quan-tum go through the left-hand slit or the right-hand slit? The answer to this question is indeterminate . There is one possible history in which it went through the one, an equally possible history in which it went through the other, and the way in which these possibilities combine follows the arith-metic of interfering wave patterns. The light quantum itself doesn’t know which slit it went through, or if it does, it’s not telling. In short, each individual photon , as the quantum of light is now called, travels as a wave, but acts as a particle, and it is the intensity of that wave that determines the probability of it acting at any one specific point. There is no saying where the photon really is , until it acts, and even then, there is no saying which slit it went through, so there is no saying where it really was before it acted. If you find this description of the world completely unsatisfactory, then you are, as we have seen, in excellent company. Particles are Waves, But No One Understands How This is Possible It gets worse. Physicists are always searching for symmetries in Nature. If light, while travelling as a wave, is emitted and absorbed in particle-like quanta, could it be that ordinary particles might show some of the proper-ties of waves? And does that include the property of being spread out in space? And if so, would that not introduce intrinsic uncertainty into the 184 From Stars to Stalagmites: How Everything Connects behaviour of all matter at very small scales? Yes, to all these questions, according to the “Copenhagen interpretation” of quantum mechanics developed by Niels Bohr and accepted by most (not all) physicists to this day.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Whenever the energy of a system can have only certain definite values, and nothing in between, the energy is said to be quantized. This quantization of the energy was unexpected on the basis of the traditional physics of the time. However, it was soon realized that energy quantization had wide-ranging and valid implications. Conservation of energy requires that the energy carried off by the radiated electro- magnetic waves must equal the energy lost by the atomic oscillators in Planck’s model. Suppose, for example, that an oscillator with an energy of 3hf emits an electromagnetic wave. According to Equation 29.1, the next smallest allowed value for the energy of the oscillator is 2hf. In such a case, the energy carried off by the electromagnetic wave would have the value of hf, equaling the amount of energy lost by the oscillator. Thus, Planck’s model for blackbody radiation sets the stage for the idea that electromagnetic energy occurs as a collection of discrete amounts, or packets, of energy, with the energy of a packet being equal to hf. Einstein made the proposal that light consists of such energy packets. 29.3 Photons and the Photoelectric Effect We have seen in Chapter 24 that light is an electromagnetic wave and that such waves are continuous patterns of electric and magnetic fields. It is not unexpected, then, that light beams, such as those in the photograph in Figure 29.3, or those coming from flash- lights, look like continuous beams. However, we must now discuss the surprising fact that visible light and all other types of electromagnetic waves are composed of discrete particle-like entities called photons. As discussed in Chapters 6 and 7, the total energy E and the linear momentum → p are fundamental concepts in physics that apply to moving particles such as electrons and protons. The total energy of a (nonrelativistic) particle is the sum of its kinetic energy (KE) and potential energy (PE), or E = KE + PE.
eBook - PDF- Kenneth S. Krane(Author)
- 2020(Publication Date)
- Wiley(Publisher)
Symbolically, photon → electron + positron This process, like bremsstrahlung, will not occur unless there is an atom nearby to supply the necessary recoil momentum. The reverse process electron + positron → photon also occurs; this process is known as electron–positron annihilation and can occur for free electrons and positrons as long as at least two photons are cre- ated. In this process the electron and positron disappear and are replaced by two photons. Conservation of energy requires that (m e c 2 + K + ) + (m e c 2 + K − ) = E 1 + E 2 (3.55) where E 1 and E 2 are the photon energies. Usually the kinetic energies K + and K − are negligibly small, so we can assume the positron and electron to be essentially at rest. Momentum conservation then requires the two photons to have equal and opposite momenta and thus equal energies. The two annihila- tion photons have equal energies of 0.511 MeV (= m e c 2 ) and move in exactly opposite directions. 3.6 PARTICLES OR WAVES In some experiments (interference and diffraction, for example), light shows a familiar wave behavior, and it is not possible to understand those experiments in any other way. In other experiments discussed in this chapter (photoelectric effect, thermal radiation, Compton scattering), light behaves as a particle, and the wave picture is not able to explain those experiments. Unfortunately, “wave” and “particle” are very different kinds of behavior and have no elements in common. A wave distributes its energy over broad wave fronts, while the energy of a particle is concentrated at a single loca- tion. The double-slit experiment can be explained only if the wave front goes through both slits, which a particle obviously cannot do. The photoelectric effect, on the other hand, could not occur if the energy of light were distributed over its wave front—the energy of the light must be concentrated in a small bundle to enable an electron to be released.
eBook - PDFInterpreting Bodies
Classical and Quantum Objects in Modern Physics
- Elena Castellani(Author)
- 2020(Publication Date)
- Princeton University Press(Publisher)
WHAT IS AN ELEMENTARY PARTICLE? – 199 phenomenon, and indeed of all physical phenomena. The union is not a loose or superficial one. It would be quite unsatisfactory to consider cathode rays to consist both of particles and of waves. In the early days of the new theory it was suggested that the particles might be singu-lar spots within the waves, actually singularities in the meaning of the mathematician. The white crests on a moderately rough sea would be a fairly adequate simile. The idea was very soon abandoned. It seems that both concepts, that of waves and that of particles, have to be modified considerably, so as to attain a true amalgamation. 3. Current Views: The Nature of Waves The waves, so we are told, must not be regarded as quite real waves. It is true that they produce interference patterns—which is the crucial test that in the case of light had removed all doubts as to the reality of the waves. However, we are now told that all waves, including light, ought rather to be looked upon as “probability waves.” They are only a mathematical device for computing the probability of finding a particle in certain conditions, for instance (in the above example), the probability of an electron hitting the photographic plate within a small specified area. There it is registered by acting on a grain of silver bromide. The interference pattern is to be regarded as a statistical registration of the impinging electrons. The waves are in this context sometimes referred to as guiding waves—guiding or directing the particles on their paths. The guidance is not to be regarded as a rigid one; it merely constitutes a probability. The clear-cut pattern is a statistical result, its definiteness being due to the enormous number of particles. Here I cannot refrain from mentioning an objection which is too ob-vious not to occur to the reader.
eBook - PDFIntroduction to Quantum Optics
From the Semi-classical Approach to Quantized Light
- Gilbert Grynberg, Alain Aspect, Claude Fabre(Authors)
- 2010(Publication Date)
- Cambridge University Press(Publisher)
At the beginning of the twentieth century, it turned out that this impressive edifice was not without its shortcomings, and that certain phenomena, such as the black-body radiation spectrum, the photoelectric effect or the Compton effect, could not be given a satisfactory expla- nation. By inventing the photon, Einstein revived the particle model, which provided a simple explanation for all these effects. But the price was high, since this model was pow- erless to account for interference and diffraction effects. There was no option but to accept that light behaved sometimes as a particle, sometimes as a wave. 18 After the advent of quantum mechanics, it was found that most of the phenomena of quantum optics, and in particular the photoelectric effect, could be accounted for by a model based on the inter- action between quantized matter and a classical electromagnetic field. It is this type of approach that was presented in Chapter 2. It was true that the photon, and more generally 18 Einstein himself stressed this problem before the development of quantum mechanics. See, for example, the text of the conference given in Salzburg in 1909: A. Einstein, Über die Entwicklung unserer Anschauungen über das Wesen und die Konstitution der Strahlung, Physikalische Zeitschrift 10, 817 (1909). 386 Free quantum radiation the quantization of light, led to simple pictures for interpreting the photoelectric effect, Compton scattering or the selection rules for atomic spectra, but the quantization of radia- tion did not appear to be absolutely essential, except for a very small number of phenomena like spontaneous emission or the Lamb shift in the hydrogen spectrum. But again, these phenomena were all concerned with radiation interacting with a microscopic source, and the quantization of free radiation seemed hardly necessary. It was not until the last few decades of the twentieth century that clarifications, due in particular to the work of R.
eBook - PDF- John Mcgervey(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
However, the easily observed par-ticle nature of the rays tended to confuse the issue. It was observed that at a distance of several meters from the point of original impact of an electron beam on a plate, another electron could suddenly appear with the same energy as the original electrons in the beam. This is, of course, simply the photoelectric effect, with X-rays instead of light. The lack of any attenuation in energy from dispersion as the X-ray traversed the intervening distance was evidence of the corpuscular nature of X-rays. Since these observations were made before Einstein's photoelectric equation had been verified for light waves, they were puzzling to many who felt that the X-rays had to be either waves or particles, but not both. The wave nature of X-rays was finally established beyond doubt when Max von Laue suggested that the atoms in a crystal should make a good dif-fraction grating for the rays, and von Laue, Friedrich, and Knipping obtained a diffraction pattern from X-rays passing through a crystal of ZnS. However, a three-dimensional crystal is considerably more complicated than the usual diffraction grating, so the pattern which was observed was difficult to inter-pret. Although von Laue gave a detailed analysis of the pattern, a clearer and more general explanation was provided by W. L. Bragg. It then became apparent that X-rays could be used to study the structure of crystals. To understand Bragg's explanation, one may visualize the atoms of a crystal as arrayed in various planes which reflect X-rays (see Fig. 1). That Fig. 1. Edgewise view of some planes formed by a cubic array of atoms. 98 WAVES AND PARTICLES is, the effect of a single plane of atoms on an X-ray is to scatter it in such a way that the usual law of reflection is obeyed: angle of incidence = angle of re-flection.
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