Physics
Photons
Photons are fundamental particles of light that carry electromagnetic radiation. They have no mass and travel at the speed of light. In quantum mechanics, photons are considered as both particles and waves, and they play a crucial role in the interaction of light with matter.
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9 Key excerpts on "Photons"
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Orange Apple(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 9 Photon Photon Photons emitted in a coherent beam from a laser Composition: Elementary particle Particle statistics: Bosonic Group: Gauge boson Interaction: Electromagnetic Symbol(s): γ, hν, or ħω Theorized: Albert Einstein Mass: 0<1×10 −18 eV Mean lifetime: Stable Electric charge: 0<1×10 −35 e Spin: 1 Parity: -1 C parity: -1 Condensed: I ( J PC ) = 0,1(1 --) In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. In other words a photon is a little packet of energy which can carry electromagnetic radiation. It is also the force carrier for the electromagnetic force. The effects of this force are easily observable at both the microscopic and macroscopic level, because the photon ________________________ WORLD TECHNOLOGIES ________________________ has no rest mass; this allows for interactions at long distances. Like all elementary particles, Photons are currently best explained by quantum mechanics and will exhibit wave–particle duality, exhibiting properties of both waves and particles. For example, a single photon may be refracted by a lens or exhibit wave interference with itself, but also act as a particle giving a definite result when quantitative momentum is measured. The modern concept of the photon was developed gradually by Albert Einstein to explain experimental observations that did not fit the classical wave model of light. In particular, the photon model accounted for the frequency dependence of light's energy, and explained the ability of matter and radiation to be in thermal equilibrium.
- 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)
CHAPTER 38 Photons and matter waves 38.1 The photon, the quantum of light LEARNING OBJECTIVES After reading this module, you should be able to: 38.1.1 explain the absorption and emission of light in terms of quantised energy and Photons 38.1.2 for photon absorption and emission, apply the relationships between energy, power, intensity, rate of Photons, the Planck constant, the associated frequency, and the associated wavelength. KEY IDEAS • An electromagnetic wave (light) is quantised (allowed only in certain quantities), and the quanta are called Photons. • For light of frequency f and wavelength , the photon energy is E = hf, where h is the Planck constant. Why study physics? Understanding radiation and how it interacts with matter helps us to understand more than just how the world works. We can explain why we feel heat from a flame, and we can capture images of bones using x-rays to identify diseases. By exploring how radiation interacts with matter, scientist have been able to develop new ways of treating diseases and have made advancements in modern medicine, such as developing new techniques to cure cancer. The photon, the quantum of light Quantum physics (which is also known as quantum mechanics and quantum theory) is largely the study of the microscopic world. In that world, many quantities are found only in certain minimum (elementary) amounts, or integer multiples of those elementary amounts; these quantities are then said to be quantised. The elementary amount that is associated with such a quantity is called the quantum of that quantity (quanta is the plural). In a loose sense, US currency is quantised because the coin of least value is the penny, or $0.01 coin, and the values of all other coins and bills are restricted to integer multiples of that least amount. In other words, the currency quantum is $0.01, and all greater amounts of currency are of the form n($0.01), where n is always a positive integer.
eBook - ePubFrom Atoms to Higgs Bosons
Voyages in Quasi-Spacetime
- Chary Rangacharyulu, Christopher J. A. Polachic, Chary Rangacharyulu, Christopher J. A. Polachic(Authors)
- 2019(Publication Date)
- Jenny Stanford Publishing(Publisher)
9 but in which no explanation is forthcoming for its emission and absorption in matter.The concept of the photon, as a discrete energy entity or packet of light, is not very different from that of Isaac Newton, who imagined light to be made up of corpuscular bullets. But in classical Newtonian thinking, there was no association between the frequency of light and its energy. In the quantum concept, this is a chief characteristic of light: the energy of a photon isE = h v (9.2)as originally given by Planck, where h is a constant of Nature. The frequency parameter for light (v ) is a continuous variable, and so is the energy that a photon may have.9.3 Waves and Particles, Real and Virtual
Until very recently, Photons occupied a unique status in particle physics. A “free” photon has zero mass, which makes the energy and momentum numerically the same value. We would recognize this as a kinetic momentum . Another kind of momentum, the canonical momentum , is defined for a photon that is not free, but present within a material medium. As Photons propagate in media, they may exert a push or pull on the material, which can be modeled as an effective mass for the photon. In this context, the canonical momentum is sensitive to the electromagnetic properties of the medium in which the photon is moving.Maxwell’s theory of electromagnetism, combined with the spacetime concepts of Einstein’s relativity as a basic premise, yields a body of theory called electrodynamics . Early developments in quantum theory incorporated Photons as waves, with quantum mechanical representations for the material bodies with which Photons might interact. Photon-matter scattering was described very well by the Klein–Nishina formula , which treats Photons as monochromatic waves of well-defined energy. Following the development of quantum electrodynamics (QED), theorists checked that new theory to see if it agreed with the successful Klein–Nishina formula, although the concept of radiation is ontologically very different in the two frameworks. In QED, matter particles and Photons do not move in space. Rather, when a photon’s position changes from point A to point B, we mathematically destroy one photon at point A and create another at point B.10 This QED process does not obey the principle of causality and leaves the physical system in an energetically unphysical state for a short time interval—an acceptable situation if we invoke Heisenberg’s uncertainty relations. An example of this kind of scenario is the well-known Compton scattering
eBook - PDF- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
1225 C H A P T E R 3 8 Photons and Matter Waves 38.1 THE PHOTON, THE QUANTUM OF LIGHT Learning Objectives After reading this module, you should be able to . . . 38.1.1 Explain the absorption and emission of light in terms of quantized energy and Photons. 38.1.2 For photon absorption and emission, apply the relationships between energy, power, intensity, rate of Photons, the Planck constant, the associated frequency, and the associated wavelength. Key Ideas ● An electromagnetic wave (light) is quantized (allowed only in certain quantities), and the quanta are called Photons. ● For light of frequency f and wavelength λ, the photon energy is E = hf, where h is the Planck constant. What Is Physics? One primary focus of physics is Einstein’s theory of relativity, which took us into a world far beyond that of ordinary experience—the world of objects moving at speeds close to the speed of light. Among other surprises, Einstein’s theory predicts that the rate at which a clock runs depends on how fast the clock is moving relative to the observer: the faster the motion, the slower the clock rate. This and other predictions of the theory have passed every experimental test devised thus far, and relativity the- ory has led us to a deeper and more satisfying view of the nature of space and time. Now you are about to explore a second world that is outside ordinary experience—the subatomic world. You will encounter a new set of surprises that, though they may sometimes seem bizarre, have led physicists step by step to a deeper view of reality.
eBook - PDF- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
1022 CHAPTER 38 Photons AND MATTER WAVES 38-5 ELECTRONS AND MATTER WAVES Learning Objectives After reading this module, you should be able to . . . 38.21 Identify that electrons (and protons and all other elementary particles) are matter waves. 38.22 For both relativistic and nonrelativistic particles, apply the relationships between the de Broglie wave- length, momentum, speed, and kinetic energy. 38.23 Describe the double-slit interference pattern obtained with particles such as electrons. 38.24 Apply the optical two-slit equations (Module 35-2) and diffraction equations (Module 36-1) to matter waves. ● A moving particle such as an electron can be described as a matter wave. ● The wavelength associated with the matter wave is the particle’s de Broglie wavelength λ = h/p, where p is the particle’s momentum. ● Particle: When an electron interacts with matter, the interaction is particle-like, occurring at a point and trans- ferring energy and momentum. ● Wave: When an electron is in transit, we interpret it as being a probability wave. Key Ideas 1023 38-5 ELECTRONS AND MATTER WAVES Electrons and Matter Waves In 1924, French physicist Louis de Broglie made the following appeal to sym- metry: A beam of light is a wave, but it transfers energy and momentum to matter only at points, via Photons. Why can’t a beam of particles have the same proper- ties? That is, why can’t we think of a moving electron — or any other particle — as a matter wave that transfers energy and momentum to other matter at points? In particular, de Broglie suggested that Eq. 38-7 ( p = h/λ) might apply not only to Photons but also to electrons. We used that equation in Module 38-3 to assign a momentum p to a photon of light with wavelength λ. We now use it, in the form λ = h p (de Broglie wavelength), (38-17) to assign a wavelength λ to a particle with momentum of magnitude p. The wavelength calculated from Eq. 38-17 is called the de Broglie wavelength of the moving particle.
eBook - PDFInformation Photonics
Fundamentals, Technologies, and Applications
- Asit Kumar Datta, Soumika Munshi(Authors)
- 2016(Publication Date)
- CRC Press(Publisher)
Light consists of Photons which are quanta of energies showing particle-like behaviour. The Photons travel at the speed of light in vacuum c 0 (henceforth it will be represented as c unless specifically mentioned otherwise). The speed of light is retarded in matter. A photon has zero rest mass but carries electromagnetic energy and momentum. It has an intrinsic angular momentum (or spin) that governs its polarisation properties. Photons also show a wavelike character that determines their localisation properties in space and therefore they interfere and diffract. 2.2.1.1 Photon energy and momentum Like each type of electromagnetic field, and like every kind of wave, light carries energy. The energy E of a photon is given as E = hν = } ω , where h = 6 . 63 × 10 -34 J-s is the Planck’s constant and } = h 2 π and ν is the frequency. The energy can be added or taken only in the units of hν . However, a resonator system having zero Photons carries an energy E 0 = 1 2 hν . Photon energy can be calculated easily. For example a photon at wave-length λ = 1 μm (frequency = 3 × 10 14 Hz) has energy hν = 1 . 99 × 10 -19 J = 1 . 24 eV . This is the same as the kinetic energy of an electron that has been accelerated through a potential difference of 1.24 V. Thus, photon energy can be converted into wavelength using a relation given by λ = 1 . 24 E , where E is in electron volts. The speed-energy dependence is also considered for Photons [20]. In gen-eral, the energy flow T per unit of surface and per unit time is given by T = 1 μ 0 E × B (2.1) where, μ 0 (= 4 π × 10 -7 N.s 2 /C 2 ) is the electric permeability of free space, and E and B are the electric and magnetic field vectors. Introduction to Photonics 39 Therefore the average energy flow is given by h T i = 1 2 μ 0 E max B max (2.2) The photon travels in the direction of wave vector and the magnitude of momentum of photon P is related to energy E by the equation P = E c (2.3) where c is the speed of light.
- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2023(Publication Date)
- Wiley(Publisher)
38.3.7 In terms of Photons, explain the double-slit experiment in the standard version, the single-photon version, and the single-photon, wide-angle version. where m is the mass of the target electron and ϕ is the angle at which the pho- ton is scattered from its initial travel direction. 5. Photons: When light interacts with matter, the interaction is particle-like, occurring at a point and transferring energy and momentum. 6. Wave: When a single photon is emitted by a source, we interpret its travel as being that of a probability wave. 7. Wave: When many Photons are emitted or absorbed by matter, we interpret the combined light as a classical electromagnetic wave. Photons Have Momentum In 1916, Einstein extended his concept of light quanta (Photons) by proposing that a quantum of light has linear momentum. For a photon with energy hf, the magnitude of that momentum is p = hf _ c = h _ λ (photon momentum), (38.3.1) where we have substituted for f from Eq. 38.1.1 ( f = c/λ). Thus, when a photon interacts with matter, energy and momentum are transferred, as if there were a collision between the photon and matter in the classical sense (as in Chapter 9). In 1923, Arthur Compton at Washington University in St. Louis showed that both momentum and energy are transferred via Photons. He directed a beam of x rays of wavelength λ onto a target made of carbon, as shown in Fig. 38.3.1. An x ray is a form of electromagnetic radiation, at high frequency and thus small wavelength. Compton measured the wavelengths and intensities of the x rays that were scattered in various directions from his carbon target. Figure 38.3.2 shows his results. Although there is only a single wavelength (λ = 71.1 pm) in the incident x-ray beam, we see that the scattered x rays con- tain a range of wavelengths with two prominent intensity peaks.
eBook - PDFQuips, Quotes and Quanta
An Anecdotal History of Physics
- Anton Z Capri(Author)
- 2011(Publication Date)
- WSPC(Publisher)
So, a single photon with enough energy (given by Planck’s constant times the frequency of the photon viewed as a wave) can eject an electron from the surface. According to this view the ejected electron would have kinetic energy equal to the difference between the photon energy and the energy that kept the electron tied to the metal. In 1909 he repeated, “It is undeniable that there is an extensive group of data concerning radiation which show that light has certain fundamental properties that can be understood much more readily from the standpoint of the Newtonian emission theory than from the standpoint of the wave theory. It is my opinion therefore that the next phase of the development of theoretical physics will bring us a theory of light that can be interpreted as a kind of fusion of the wave and emission theory.” The term “photon” is due to the physical chemist Gilbert Newton Lewis (1875 – 1946) who also suggested the name “jiffy” for the time it takes light to travel one centimeter. Robert Millikan (1868 – 1953) verified Einstein’s explanation of the pho-toelectric effect in the period 1912 to 1915. It was an exceedingly difficult experiment because the energy with which the electron is bound to the surface of the metal depends very sensitively on any surface contamination. 110 Quips, Quotes, and Quanta: An Anecdotal History of Physics You need very clean surfaces to get a reproducible result. Millikan solved this by enclosing a chunk of sodium metal, which is very soft, inside a vac-uum. He used a razor blade that he could manipulate from outside the vacuum to shave a thin slice off the surface after each measurement so that the Photons always hit a fresh surface.
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.
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