The Photon Theory of Light “Life can only be understood backwards, but must be lived forwards” Søren Kierkegaard (Danish philosopher noted in his diary in 1843 “---- knowledge must precede application, and the more detailed our knowledge -----, the richer and more lasting will be the results which we can draw from that knowledge.” Nobel Prize, 1918 “without taking account of its historical development, an existing theory often appears almost as if it had "fallen from heaven. However, the question of the development of a theory is important not only to satisfy our curiosity but also because much can be learnt from it for the future”. G. Ludwig, Wave Mechanics, Pergamon Press, Oxford (1968) A historical account of the photon theory of light Thirty Years that Shook Physics, The Story of Quantum Theory, George Gamow, 1965, Ch 1, pp 6-28. The Great Physicists from Galileo to Einstein, George Gamow, 1961, Ch 7, pp. 225-236. Introducing Quantum Theory, A Graphic Guide, J. P. McEvoy and O. Zarate, 2013. Boltzmann's atom: the great debate that launched a revolution in physics, D. Lindley, 2001 Catching the Light, The Entwined History of Light and Mind, A. Zajonc, 1993. (selected chapters) Electrodynamics from Ampere to Einstein, Olivier Darrigol, Oxford Uni Press, 2000. Theoretical Concepts in Physics, 3rd ed. Malcolm Longair, Cambridge Uni Press, 2020. Reference Books Reference Articles • The Nature of Light, Max Planck, A Survey of Physical Theory, Dover Publications, 1960, p. 89. • The Origin and Development of the Quantum Theory, Max Planck, A Survey of Physical Theory, Dover Publications, 1960,. p.102. • The History of Modern Physics: Vol 12, The Conceptual Development of Quantum Mechanics, Max Jammer, American Institute of Physics, 1989, pp 1-64. • The Story of the Photon, N. Mukunda, Resonance, March 2000, p. 35. • Black-Body Radiation, G. S. Ranganath, Resonance, Feb 2008, p. 115. • The Particle Nature of Radiation: Photon, Optics, 6th ed. A. Ghatak, Mc Graw Hill, 2017, Ch 25, p. 25.3 Modern science starts with Greeks Socrates, Plato, and Aristotle The Ancient Greeks are seen, in the west, as our intellectual forefathers. From Greece was born philosophy, drama, western artistic aesthetics, geometry, etc., etc. Matter is made up of four elements Five interconvertible elements make up the earth Pancha Bhoota (Sanskrit: प"चभूत), five great elements, which, according to Hinduism, is the basis of all cosmic creation. • Prithvi (Sanskrit: पृ+वी:, Earth), • Jal (Sanskrit: अप:, Water), • Agni (Sanskrit: अि1न, Fire), • Vayu (Sanskrit: वायु:, Air), • Akasha (Sanskrit: आकाश, Space ). The Indian View of the Universe (Taittirīya Upanishad and Aitareya Upanishad, 6th century BC) Asians and Arabs had their own ideas • Light has no taste, no smell and no gravity. • Light and heat are different forms of the same, Tejas. • Light (Tejas) is constituted by infinitely small particles, something smaller and different than ‘anu or paramanu’ (atoms). • These particles radiate themselves in all directions from their source and with inconceivable velocity. • Light is the root, the nourisher, and the supporter of the tree of life. Indian View: Tejas, Light and Fire Vaiśeṣika Sūtra Acharya Kanada (Kashyap) The Father of Atomic Theory Vatsayana, The Nyaya School Between 6th and 2nd century BCE Greek View: Light is made up of particles Democritus c. 460 – c. 370 BC Vision occurs by means of the images flowing from objects. We see by the impact of images on the eye. Sunlight is presumably, like fire, composed of small, swift-moving round atoms. The air through which the object’s image moves is infused with light particles from the sun and is imprinted on the eye. Euclid's Optics, 300 BC Greek View: Light is made up of particles “The light and heat of the sun are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove.” Euclid, 300-265 BC • According to Euclid, the eye sees objects that are within its visual cone. The visual cone is made up of straight lines, or visual rays, extending outward from the eye. Visual rays is a cone of which the vertex is at the eye and the base at the surface of the objects seen. • These visual rays are discrete, but we perceive a continuous image because our eyes, and thus our visual rays, move very quickly. • That those things are seen upon which visual rays fall and those things are not seen upon which visual rays do not fall. Euclid’s view of vision: Eye to the object In the beginning God created the heavens and the earth. Now the earth was formless and empty, darkness was over the surface of the deep, and the Spirit of God was hovering over the waters. And God said, "Let there be light," and there was light. God saw that the light was good, and he separated the light from the darkness. God called the light "day," and the darkness he called "night." Genesis 1:3-5 Middle eastern ancient (Christian) view of the universe Ḥasan Ibn al-Haytham 965-1040 AD Book of Optics 7 volumes Arabic view: Ḥasan Ibn al-Haytham on vision Eye to brain Democritus c. 460 – c. 370 BC Euclid ~300-265 BC Vision occurs when light reflected from an object passes to one's eyes. Light travels in straight lines The object sends an infinite number of rays of light to the eye, only one of these lines falls on the eye perpendicularly. All the rays other than the one that hits the eye perpendicularly are not involved in vision Ḥasan Ibn al-Haytham on light and vision Vision occurs in the brain, rather than in the eyes. https://www.youtube.com/watch?v=4Dk2CfO5PAY https://www.youtube.com/watch?v=YUpoccBrAc0 Ḥasan Ibn al-Haytham on light and vision https://www.youtube.com/watch?v=SxJ2OC7iXo0 Ben Kingsley Smithsonian al-Haytham Which is correct, how would you verify? “eyes receive light reflected from objects, rather than emanating light themselves” --- Ḥasan Ibn al-Haytham “light emanates from eyes & hits the objects” --- Euclid EuclidḤasan Ibn al-Haytham Pierre Gassendi (French) 1592–1655 AD Light is composed of corpuscles (particles of matter) which are emitted in all directions from a source. Light particles travel at unimaginably highspeed (qualitatively correct). Vision is a function of rays of light atoms or image-bearing atoms that are received by our internal apparatus for vision. European view: What is Light Ḥasan Ibn al-Haytham 965-1040 AD 1596 –1650 AD René Descartes (French) • The universe is filled with some material (named ‘plenum’), which pressed against the eyes. This pressure produces the phenomenon of sight. A bright object, like the Sun, generates pressure and that instantaneously is felt by the human eye. • Objection: If sight is caused by the pressure of the plenum on the eye, then a person running at night should be able to see, because the runner’s motion would make the plenum press against their eyes. European view: What is sight? René Descartes, The World and Other Writings, English Translation, Cambridge Uni Press, 2004 • Allowed a beam of sunlight to pass through a small aperture in a screen and noticed that it was diffused in the form of a cone. The shadow of a body placed in the path of the beam was larger than that required by the rectilinear propagation of light. • Light is propagated or scattered not only directly and by reflection and refraction, but also in a certain other mode, namely by diffraction. • Diffraction describes the event of waves encountering an obstacle and the consequential bending around the object. Francesco Maria Grimaldi (Italian) 1618 –1663 Light diffracts: It is a wave Light diffracts Christian Huygens (Dutch) 1629 to 1695 In 1678, Huygens proposed every point on a wavefront is a source of wavelets that spread out in the forward direction at the same speed as the wave itself. The sum of these secondary waves determines the form of the wave at any subsequent time. The new wavefront is a line tangent to all of the wavelets. Francesco M. Grimaldi (Italian) 1618 –1663 Diffraction is the bending of a wave around the edges of an opening or other obstacle. The edges of the wavefront bend after passing through the opening. The amount of bending is more extreme for a small opening, consistent with the fact that wave characteristics are most noticeable for interactions with objects about the same size as the wavelength. Light bends around corners For plane waves entering a single slit, the waves emerging from the slit start spreading out, diffracting. The extent of spread depends on the slit opening size. • Amplitude – for any diffraction to occur, the incident waves must have a higher amplitude than the slit width. If the wave is smaller than the slit width, no diffraction will occur. • Slit Width – Assuming an incident plane wave, decreasing the slit width will make diffraction more dramatic, and increase the angle at which the waves spread from the slit. • Wavelength – Decreasing wavelength, or increasing frequency has a similar effect as increasing the slit width. A lower wavelength decreases the diffracted angle Factors controlling light bending around corners Mechanism of diffraction Same amplitude and different wavelengthDifferent amplitude and wavelength Factors controlling light bending around corners • Light passing through a doorway makes a sharp outline on the floor. Since light’s wavelength is very small compared with the size of the door, it acts like a ray. • (b) Sound waves bend into all parts of the room, a wave effect, because their wavelength is similar to the size of the door. Difference between light and sound “Others have seen him (Sun) riding in wisdom on his chariot, with seven colors as horses and six wheels to represent the whirling spokes of time”. The Prashna Upanishad (1-4 century BC) Rain drops, rainbow and colors In the 1660s, Newton began a series of experiments with sunlight and prisms. He demonstrated that clear white light was composed of seven visible colors. “If the Sun’s Light consisted of but one sort of Rays, there would be but one Color in the whole World”. Newton (1660s) In 1665, Isaac Newton, took a glass prism and held it up to a beam of sunlight streaming through the window. He saw the sunlight that passed through the prism spread out into the colors of the rainbow red, orange, yellow, green, blue and violet. Thus he showed white light is in fact a combination of seven visible colors. In 1800, William Herschel discovered a form of light (or radiation) beyond red light, now known as infrared radiation. Herschel measured the temperature just beyond the red portion of the spectrum in a region apparently devoid of sunlight. To his surprise, he found that this region had the highest temperature of all. This region is now known as infrared (IR). In 1801, J. W. Ritter made a significant discovery while investigating rays beyond the violet color in the light spectrum. During his experiments, Ritter observed that these rays were capable of darkening paper impregnated with silver chloride. They are now known as ultraviolet (UV) and due have the ability to induce chemical reactions. Light is impure-a mixture of visible and invisible parts Sir Isaac Newton (1643-1727) Corpuscular theory of light Light is a particle because the periphery of the shadows it created was extremely sharp and clear. Newton argued that the geometric nature of reflection and refraction of light could only be explained if light were made of particles, because waves do not tend to travel in straight lines. Every source of light emits large numbers of tiny particles that are elastic, rigid, and weightless. Light travels in a straight line: It is a particle 1596 –1650 AD René Descartes Pierre Gassendi 1592–1655 AD Francesco M. Grimaldi 1618 –1663 Christian Huygens 1629 to 1695 Sounds are propagated as readily through crooked pipes as through straight ones. But light is never known to follow crooked passages nor to bend into the shadow. Isaac Newton 1643-1727 al-Haytham 965-1040 AD 1596 –1650 AD René Descartes Thomas Young 1773 - 1829 Light is a particle ~500 BC to 1650 Francesco M. Grimaldi 1618 –1663 Christian Huygens 1629 to 1695 Robert Hooke 1635-1703 Democritus 460 – 370 BC Euclid 300 –265 BC Kanada (Kashyap) 600-400 BC Pierre Gassendi 1592–1655 AD Light is a wave ~ 1650s Robert Hooke 1635-1703 • Hooke opined that a combination of different amounts of red and blue originated the rest of the colors. This is different from Newton’s suggestion that light is made up of seven coloors. • Hooke proposed light is a wave because it diffracts while Newton said it is a particle.” Micrographia (1665) Hooke-microscope Hooke was an early one to build and experiment with microscopes Hooke (1635-1703) vs Newton (1643-1727) What is light, particle or wave? Because of Newton’s enormous prestige, his support of the particle theory of light tended to suppress other points of view. This continued for 100 years. In 1678 Christian Huygens argued that light was a pulse traveling through a medium, a wave. Treatise on Light (French) 1690 In the late 1672 Newton explained many of the properties of light by assuming it was made of particles. 1704 What is light? Kanada, Democritus, Euclid, Gassendi, Newton: light is composed of a large number of particles Particles Grimaldi, Hooke and Huygens: light is composed of waves wavelength Electromagnetic wave Light is ---- Seven colors that we can see (1665) Seven colors plus one beyond red (infra-red) that we can’t see (1800) Isaac Newton William Herschel Seven colors plus one beyond violet (ultraviolet) that we can’t see (1801) J. W. Ritter French physicist Augustin-Jean Fresnel asserted in 1815 that light is a wave and mathematically proved the phenomenon of light interference. He also hypothesized that space is filled with a medium known as ether because waves need something that can transmit them. Augustin-Jean Fresnel (1788 to 1827) Light is a wave With interference experiment, or double-slit experiment, in 1817 Young demonstrated interference in the context of light as a wave. Thomas Young (1773 to 1829) Light is a wave: A new evidence Thomas Young (1773 to 1829) (~200 yrs ago) Double-slit experiment demonstrated interference in the context of light as a wave The bright and dark bands demonstrated that the slits were causing light waves to interfere with each other. Sometimes this interference is constructive and sometimes destructive. This leads to light waves adding together to create a bright patch and cancelling each other out creating dark patches. Interference of light is a common phenomenon The interference of two waves. When in phase, the two waves create constructive interference resulting in a wave of greater amplitude. When 180° out of phase, they create destructive interference. SubstractAdd Web links https://www.olympus-lifescience.com/en/microscope- resource/primer/java/polarizedlight/emwave/ https://www.olympus-lifescience.com/en/microscope- resource/primer/lightandcolor/ https://www.youtube.com/watch?v=ak7GB74Qlug Democritus 460 – 370 BC Euclid 300 –265 BC al-Haytham 965-1040 AD Pierre Gassendi 1592–1655 AD Francesco M. Grimaldi 1618 –1663 Christian Huygens 1629 to 1695 Isaac Newton 1643-1727 Kanada (Kashyap) 600-400 BC 1596 –1650 AD René Descartes Thomas Young 1773 - 1829 Wave or particle remains unresolved 500 BC to 1800 AD Robert Hooke 1635-1703 The Electric Life of Michael Faraday, A. Hirshfield, 2006 Faraday Rediscovered, D. Gooding and F. A. J. L. James, 1985 Faraday, Maxwell, and the Electromagnetic Field: How Two Men Revolutionized Physics, N. Forbes and B. Mahon, 2014. Michael Faraday https://royalsociety.org/science-events-and-lectures/2017/12/genius-legacy-michael-faraday/ Lecture by Sir John Thomas What is light made up of? • Born near London on Sep 22 1791; three siblings • Father a blacksmith, mostly unemployed and unhealthy • Mother from a family of farmers • Deeply religious, Sandemanian sect of Christianity • Educated in rudimentary reading, writing and arithmetic, age 5-13 • Took up a job at age 13 as an errand-boy and bookbinder for a local shopkeeper (Mr. Riebeau) • At the age of 18 he has to care for the family and took up a job under Humphry Davy in London The Beginning •The Encyclopedia Britannica – his source for electrical knowledge and much more •Conversations on Chemistry – 600 pages of chemistry for ordinary people written by Jane Marcet based on Davy’s lectures at the Royal Institution “the most wonderful and the most interesting phenomenon of nature are almost all of them produced by chemical powers” Conversations on Chemistry, 1817 Jane Marcet, 1769-1858 Davy’s lectures at RI recorded Faraday, a bookbinder's apprentice at the time, lacked a formal education and studied by reading books and attending lectures. He attended around 13 lectures by silversmith John Tatum (1772- 1858) between February 1810 and September 1811. The notes Faraday made from these lectures formed four volumes and 300 pages and helped him start his career in science. John Tatum’s lectures (1772-1858) exposed Faraday to science Tatum founded the City Philosophical Society in 1808 where Faraday and other local people received inspiration. “My desire to escape from trade, which I thought vicious and selfish, and to enter into the service of Science, which I imagined made its pursuers amiable and liberal, induced me at last to take the bold and simple step of writing to Sir H. Davy, expressing my wishes, and a hope that if an opportunity came in his way he would favor my views; at the same time, I sent the notes I had taken of his lectures.” (1812) Based on the letter Faraday wrote about the experience later in life after Davy’s death to J.A. Paris. (1829) Getting Started “Sir,– I am far from displeased with the proof you have given me of your confidence, and which displays great zeal, power of memory, and attention. I am obliged to go out of town and shall not be settled in town till the end of January; I will then see you at any time you wish. It would gratify me to be of any service to you; I wish it may be in my power. I am, Sir, your obedient humble servant, H. Davy.” Davy’s encouraging reply to Faraday Michael Faraday establishes that light is electromagentic 1791-1867 The First Electromagnetic Induction The basis of electric power generation Series of pioneering discoveries during 1821-1831 Electricity to Magnetism Magnetism to Electricity In the iron ring experiment the current in coil ‘A’ produces magnetism in the ring. While this magnetism is being produced, and consequently is in motion, it is able to produce a current in coil ‘B’. When the magnetism is raised to a steady state it becomes stationary and the current in coil B disappears. A crude observation into a full-blown discovery. World’s First Electric Motor-1821 Electromagnetic rotation experiment of Faraday Wire fixed Magnet free Magnet fixed Wire free Gift model Exploratory Experiments, Ampere, Faraday and the Origins of Electrodynamics, F. Steinle & A.Levine, 2005 • A stationary magnet is a static phenomenon. Hence a stationary magnet cannot produce the converse of a steady current. If moving electricity produces magnetism, then moving magnetism will be necessary to produce electricity. • It was Faraday's discovery that a changing magnetic force was needed. Relative motion is important Changing magnetic fields produced electric fields Consequences of invention of electromagnetic induction Electrical Generator, Dynamo, Transformer Magnetic lines of force A struggle to conceptualize how forces could be transmitted across empty space. If iron filings are sprinkled around a magnet, they form discrete curves, originating at the poles and extending into the space surrounding the magnet. Faraday’s Lines of Forces The basis of electricity and magnetism and do they communicate? Magnetic and electrical lines of force are curved, not straight lines. Faraday’s view Field Newton’s view Action at a distance What is a field----does anyone know? “How few understood the physical lines of force.” Faraday to his nice, 1855 William Thomson (Lord Kelvin) 1824-1907 Interaction of magnetic field and light In 1845, Thomson mathematically analyzed Michael Faraday’s magnetic lines of force and wrote a letter to him in August of that year explaining how his calculations predicted that magnetic fields should affect the plane of polarized light. Faraday had many years before experimented with light and magnetism, but without observing any connection between the two. Encouraged by Thomson’s prediction, Faraday decided to readdress the problem and began a new series of experiments in his laboratory. Faraday Effect: Magnetic field rotates plane polarized light Light is an electromagnetic wave In 1845, Michael Faraday discovered that the plane of polarization of linearly polarized light is rotated when the light rays travel along the magnetic field direction in the presence of a transparent dielectric, an effect now known as Faraday rotation Michael Faraday 1791-1867 Effect of Electricity and Magnetism on Light “I happen to have discovered a direct relation between magnetism and light, also electricity and light---and the field it opens is so large & I think rich that I naturally wish to look at it first” To Ampere, Nov 1845 What is light? A link between magnetism, electricity and light established Michael Faraday 1791-1867 “I am inclined to compare the diffusion of magnetic forces from a magnetic pole to the vibrations upon the surface of disturbed water or those of air in the phenomenon of sound; i.e. I am inclined to think the vibratory theory will apply to these phenomena, as it does to sound and, most probably, to light.” In March 1832, Faraday asked the secretary of the Royal Society to deposit a note in his safe. Claiming the precedence: Establishing for the first time a link between magnetism and light This proposal is very close to saying light propagates as waves not in straight lines. This idea is different from Newton’s theory of action at a distance. Light travels in straight lines. Faraday Lines of force If you look for exact knowledge in his theories you will be disappointedflashes of wonderful insight you meet here and there, but he has no exact knowledge himself, ---- Biot Faraday should brush up on his mathematics and leave theoretical physics to the properly trained. Airy Faraday rejected several long-held Newtonian assumptions. • dismissed action at a distance. • scrapped the need for the ether. Faraday lacked the classical education of his colleagues, his theories forged a new path away from traditional Newtonian force relations. He moved away from traditional scientific thought of his day The Man who Changed Everything: The Life of James Clerk Maxwell, Basil Mahon, 2003 Faraday, Maxwell, and the Electromagnetic Field: How Two Men Revolutionized Physics, N. Forbes and B. Mahon, 2014. “One scientific epoch ended and another began with James Clerk Maxwell” A. Einstein “From a long view of the history of mankind seen from, say, ten thousand years from now there can be little doubt that the most significant event of the nineteenth century will be judged as Maxwell’s discovery of the laws of electrodynamics.” R. Feynmann James Clerk Maxwell (1831-1879) Men of Science, J. G. Crowther, 1936, Ch. 5. James Clerk Maxwell Einstein’s Heroes: Imagining the World through the Language of Mathematics, R. Arianrhod, 2005 Knowns and unknowns about light before Maxwell • Light responds to electricity and magnetism (Faraday rotation) • Light is a mixture of various visible and invisible radiations (visible, UV and IR radiations) • Light travels like a wave (double slit expt); particle idea dropped • Electric field generated between two metal plates and magnetic field generated by magnets are independent of each other. • Electricity and magnetism creates fields and don’t follow the Newtonian principle of ‘action at a distance’ (Faraday) Knowns Unknowns • The speed of light • Connection between electric and magnetic fields • Connection between electric and magnetic fields and light • Likely existence of other rays of light in addition to visible, UV and IR James Clerk Maxwell (1831-1879) • In 1861, Maxwell extended Faraday’s proposal by mathematically deriving that changing electric fields produced magnetic fields and in fact the two phenomena should be perceived as a single entity. • This means oscillating electric fields would produce magnetic fields. Oscillating magnetic fields would produce electric fields. • A moving electric charge would thus produce a magnetic field. Maxwell’s Equations of Electromagnetism Gauss’ Law for Electrostatics Gauss’ Law for Magnetism Faraday’s Law of Induction Ampere’s Law Maxwell explained electric and Magnetic fields in mathematical equations • Electric charges produce electric fields (Coulomb’s Law) • Electric currents (moving charges) produce magnetic fields (Ampere’s Law) • An electromagnetic wave is a combination of electric and magnetic fields that vibrate together in space and time in a synchronous fashion and propagate at the speed of light • Maxwell’s equations showed that electricity and magnetism are two sides of the same coin, and that light is that coin in movement. The generation of an electromagnetic wave wave emitter e.g. antenna electric field magnetic field • The electromagnetic wave is a transverse wave, the electric and magnetic fields oscillate in the direction perpendicular to the direction of propagation • The time varying electric field generated the time varying magnetic field which generates the time varying electric field and so on and so on . . . . • The electric and the magnetic part stimulate each other producing a cycle. Maxwell predicts the speed of light Where the speed of light “c” is given through constants from both electricity and magnetism. Maxwell’s Laws Four equations describe the behaviors of electricity and magnetism 1. Coulomb’s Law of static electricity 2. All magnets have both north and south poles 3. Electricity produces magnetic effects 4. Moving magnets produce electricity These equations lead to prediction of waves: 1. Waves travel 186,000 miles per second 2. Light is a consequence of electricity and magnetism switching back and forth James Clerk Maxwell Light is a traveling electromagnetic wave Unified electromagnetism and light. Explained the existence of invisible forms of light. Electromagnetic waves propagate in free space at c = 3 x 108 m/s. E and B are always perpendicular to each other, and perpendicular to the direction of propagation. Based on the double slit experiments of Thomas Young and the equations of Maxwell, by 1900 most scientists believed that light behaved as a wave. Maxwell: Light is an electromagnetic wave Maxwell to Faraday, October 1861 "I think we have now strong reason to believe, whether my theory is a fact or not, that the luminiferous and the electro-magnetic medium are one. In other words, light is indeed an electromagnetic undulation-a rayvibration, as you had called it in 1846.” “The electromagnetic theory of light, as proposed by Faraday, is the same in substance as that which I have begun to develop in this paper, except that in 1846 there were no data to calculate the velocity of propagation.” Maxwell Publication in 1865 https://www.youtube.com/watch?v=WqefMRAxt2k “I happen to have discovered a direct relation between magnetism and light, also electricity and light---and the field it opens is so large & I think rich that I naturally wish to look at it first” Faraday to Ampere, Nov 1845 Summary: The Laws of Electricity and Magnetism • Laws of electricity • Electric charges produce electric fields (Coulomb) • Electric fields begin and end on charges • Laws of magnetism • Currents produce magnetic fields (Ampere) • Magnetic field lines are closed loops • A changing magnetic field can produce a current (induced currents) (Faraday) • A changing electric field can produce a magnetic field (Maxwell) Heinrich Rudolf Hertz • 1857 – 1894 (lived for 37 yrs) • German physicist • First to generate and detect electromagnetic waves in a laboratory setting in 1887. • As predicted by Maxwell he established the existence of radiowaves. • Established the phenomenon of photoelectric effect Hertz 1857-1894 Accidental Discovery A great number of modern developments, like radio, television and Wi-Fi were spun out of Hertz’s simple demonstrations. Experimental support for light as wave "I do not think that the wireless waves I have discovered will have any practical application." Ø Sparks were induced across the gap of the receiving electrodes when the frequency of the receiver was adjusted to match that of the transmitter. Ø Hertz thought that if Maxwell was right, this would radiate electromagnetic waves through air. In a series of other experiments, Hertz also showed that the radiation generated by this equipment exhibited wave properties. Ø Interference, diffraction, reflection, refraction and polarization Ø He also measured the speed of the radiation. Ø It was close to the known value of the speed of light. Hertz accidently discovered in1888 • Faraday laid the groundwork with his discovery of electromagnetic induction. (1846) • Maxwell explained theoretically that light is an electromagnetic wave (1865) • Heinrich Hertz showed experimentally that EM waves exist travels at the speed precited by Maxwell. (1887) Light is indeed electromagnetic waves James Clerk Maxwell 1831-1879 Heinrich Rudolf Hertz 1857-1894 Michael Faraday 1791-1867 • Hans Christian Oersted finds that an electric current deflects a compass needle. (1820) • Andre Marie Ampère finds that parallel wires carrying current produce forces on each other. (1820) Paul Villard (1860 - 1934) F. W. Herschel 1738-1822 Infrared (1800) J. W. Ritter 1776-1810 Ultraviolet (1801) H. R. Hertz (1857-1894) Radiowaves (1886) Over and Beyond the Rainbow W. C. Röntgen (1845 - 1923) J. C. Bose (1858 - 1937) Microwaves (1894) Visible rays (1665) Isaac Newton X-Rays (1895) g-Rays (1900 • The amplitude is the wave’s height from the origin to a crest. The Wave Nature of Light • The frequency (n) is the number of waves that pass a given point per second. • Wavelength and frequency are inversely related—the shorter the wavelength, the higher the frequency. • Light is a type of energy that travels through space at a constant speed of 3.0 × 108 m/s (186,000 mi/s). • Classical: Energy carried by a light wave is proportional to the Amplitude of wave. The Wave Nature of Light Amplitude and Wavelength Uses of electromagnetic radiations of different wavelengths l = c/n n = c/l How does light wave travel? • Ocean waves water molecules • Sound molecules in air • Light plenum later aether Believed that an invisible substance, called the plenum, permeated the universe. Light is a disturbance that traveled through the plenum. Plenum was changed to aether and Thought to be the medium through which light propagates. The universe is filled with a fluid called aether. This idea was supported by numerous scientists René Descartes 1596 –1650 End of aether came due to Albert Einstein’s theory of relativity (1905) Robert Boyle Christiaan Huygens J. C. Maxwell H. Hertz H. Lorentz Speed of Energy Transmission Democritus 460 – 370 BC Euclid 300 –265 BC al-Haytham 965-1040 AD Pierre Gassendi 1592–1655 AD Francesco M. Grimaldi 1618 –1663 Christian Huygens 1629 to 1695 Isaac Newton 1643-1727 Kanada (Kashyap) 600-400 BC 1596 –1650 AD René Descartes Thomas Young 1773 - 1829 Wave or particle remains unresolved 500 BC to 1800 AD Robert Hooke 1635-1703 Michael Faraday 1791-1867 James Clerk Maxwell 1831-1879 Heinrich R. Hertz 1857-1894 W. C. Röntgen (1845 - 1923) Paul Villard (1860 - 1934) J. W. Ritter 1776-1810 F. W. Herschel 1738-1822 J. C. Bose (1858 - 1937) G. Marconi (1874 - 1937) K. F. Braun (1850 - 1918) Light is an electromagnetic wave Light is a wave. Faraday experimentally showed that electricity and magnetism are related. Faraday proposed light is electromagnetic. Maxwell theoretically established the connection between electricity and magnetism and predicted the speed. Hertz established that there are electromagnetic waves that we can’t see and many were identified. Hertz measured the speed of electromagnetic wave and confirmed Maxwell’s prediction. Light travels at different frequencies but all the same speed. What we know thus far Heat is Light Hot solid bodies give out radiation Light bulb filament Electric heating element Cook-out grill Sun Generation of Light Temperature and color distribution of galaxy Color of the star is related to surface temperature Constellation of Orion Why are the planets in our solar system so different in colors? An object at any temperature is known to emit thermal radiation. The thermal radiation consists of a continuous distribution of wavelengths from all portions of the electromagnetic spectrum. A black body is an ideal system that absorbs all radiation incident on it. “A good absorber is a good emitter” (Kirchhoff) A material with emission/absorption (E/A) as ONE is called ‘Black Body’. An example of such a thing would be an enclosed cavity, the internal surface of which continuously emits and absorbs radiation of all frequencies. Kirchoff and Blackbody Radiation Gustav R. Kirchhoff 1824 –1887 Apparatus of Lummer and Kurlbaum to measure the spectrum of black-body radiation. An electrical current heats the filament E located in a tube inside the cylinder C to a fixed temperature T, giving rise to blackbody radiation inside this cylinder. The spectrum of this radiation is observed by some radiation exiting through the hole at one end along the axis of the cylinder. Black body is a hypothetical perfect radiator that absorbs all incident light and, therefore, emits all of that light when maintained at a constant temperature in order to preserve equilibrium. The Bunsen-Kirchhoff Spectroscope with Bunsen Burner (1859) A) Box, colored black on the inside; (B) & (C) Telescopes; (D) Bunsen Burner; (E) Sample Holder; (F) Prism; (G) Mirror; (H) Handle to rotate prism and mirror. The first UV-Vis absorption spectrometer The spectrum (wavelength and intensity) depend on the temperature. The hotter the black body, the shorter the peak wavelength. A black body emits all wavelengths of light but not equally; there is always a wavelength in which the radiation peaks. The peak height increases and shifts to shorter wavelengths as the temperature of the black body increases. Black-Body Spectrum at different temperatures Robert Kirchhoff, 1860 where is a constant (the Stefan-Boltzmann constant) which has a value of and T is the absolute temperature (in Kelvin) E = T 4 The Stefan-Boltzmann Law Connects temperature to the amount of energy released 5.67×10−8 W⋅m−2⋅K−4 m = a/T where λ m is the WAVELENGTH in the spectrum at which the energy peak occurs T is the absolute TEMPERATURE of the body, and a is a constant (with a value of 2898) (if λ m is expressed in micrometers.) Wien’s Law The hotter the body, the shorter the wavelength The cooler the body, the longer the wavelength Connects temperature to the maximum of emission wavelength Wien displacement law, useful for instance in determining the temperature of the sun and stars. As per the equation the Sun’s temperature is calculated to be 5700 K. The discovery was awarded Physics Nobel Prize in 1911. Another attempt to fit the spectrum The Lord Rayleigh Nobel Prize 1904 Rayleigh-Jeans law The theory did match experimental data for longer wavelengths but failed miserably for shorter ones. This is known as the “Ultraviolet Catastrophe,” a name given by Paul Ehrenfest in 1911. Wien's Law Rayleigh-Jeans law lpeak= {hc/(4.965 k)}/T Comparison of Rayleigh–Jeans law with Wien approximation and Planck's law, for a body of 5800 K temperature. Where is the problem? Wien formula fits the shorter wavelength but fails the longer wavelength side of the spectrum. Rayleigh-Jeans formula fits the longer wavelength but fails at shorter wavelength but fits at longer wavelength Both formulae were developed using the well accepted classical physics principle—equipartition of energy (1/2kT). Equipartition Theorem (Classical Physics) Every particle has a translational energy of “The total energy contained in the assembly of a large number of individual particles exchanging energy among themselves through mutual collisions is shared equally (on the average) by all the particles.” The theorem of equipartition of energy states that molecules in thermal equilibrium have the same average energy associated with each independent degree of freedom of their motion. Energy Clausius and other physicists of 1800s had imagined all the atoms in a gas moving at the same speed. They knew this wasn’t true, that in fact atoms would move with a range of speeds, but they didn’t have the mathematical sophistication to tackle the full problem. Motion Maxwell-Boltzmann Distribution Statistic/Probabilistic Approach L. Boltzmann (1844-1906) James Clerk Maxwell (1831-1879) “The true logic for this world is the Probabilities. … This branch of Math., which is generally thought to favor gambling, dicing, and wagering, and therefore highly immoral, is the only ‘Mathematics for Practical Men,’ as we ought to be.” Maxwell, 1850 Temperature dependence of velocity of molecules: Maxwell-Boltzmann distribution Temperature dependence of wavelength of emitted light: Blackbody radiation Planck noticed the similarity Planck’s Theory of Blackbody Radiation • In 1900 Max Planck developed a theory of blackbody radiation that leads to an equation that correctly predicted the intensity and wavelength of the radiation with temperature. • To achieve this, he moved away from well established principles of classical physics, equipartition theorem. • For the first time he analyzed the data based on probabilities (modern statistical physics due to Boltzman) that was looked down upon at that time. Max Planck Planck’s Assumptions The absorption and emission are done by oscillators present in the blackbody. The oscillators emit or absorb energy when making a transition from one state to another. The oscillators with E/A of one generate standing waves of different wavelengths depending on the temperature. Absorption and emission wavelengths are quantized. They are not done in piecemeal; i.e., oscillator states can’t be populated by several partial absorptions in steps. The absorption and emission energies of an oscillator can have only certain discrete values En. It is not continuous as in x = 1, 2, 3 etc;. but jumps as in nx where for eg. n=2 and x is 1, 2, 3; then the numbers are 2, 4, 6. Notice they are not continuous. According to Planck distribution of energies follows the equation below E = hv v is the frequency of oscillation h is Planck’s constant (note v is multiplied by a constant h) . • B = Spectral density of electromagnetic radiation emitted by a black body; the cgs unit erg·s−1·sr−1·cm−2·Hz−1. • h = Planck’s constant = 6.63 ×10-34 Joule - seconds • k = Boltzmann’s constant = 1.38 ×10-23 Joule -K-1 • c = velocity of light = 3 ×10+8 meter - seconds-1 • T = temperature [K] • l = wavelength [meters] ( ) 2 5 2 1 1 hc kT hc B T el l = Planck’s Radiation Law Classical View Compared to Quantum Mechanical View Classical View Quantum Mechanical View Any energy is possible Only a few energy states are allowed Planck’s Model Summary of Blackbody Radiation • Classical BB presents a “ultraviolet catastrophe” • The spectral energy distribution of electromagnetic radiation in a black body can’t be explained in terms of classical Maxwell EM theory, in which the average energy in the cavity assumes continuous values of = kT • To solve the BB catastrophe one has to assume that the energy of individual radiation oscillator in the cavity of a BB is quantized as per En = nhν. One photon crarries energy of hν and n photons nhν. • This picture is in conflict with classical physics because in classical physics energy is in principle a continuous variable that can take any value between 0 ॠ• One is then lead to the revolutionary concept that ENERGY OF AN OSCILLATOR IS QUANTISED Max Planck "in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta." The Nobel Prize in Physics 1918 “Planck’s radiation theory is, in truth, the most significant lodestar for modern physical research, and it seems that it will be a long time before the treasures will be exhausted which have been unearthed as a result of Planck’s genius.” Nobel Prize Award ceremony speech by President of the Royal Swedish Academy of Sciences, on June 1, 1920 Planck’s radiation theory is, in truth, the most significant lodestar for modern physical research, and it seems that it will be a long time before the treasures will be exhausted which have been unearthed as a result of Planck’s genius. Nobel Prize Award ceremony speech by President of the Royal Swedish Academy of Sciences, on June 1, 1920 This marked a turning point in the history of physics. The importance of the discovery although not appreciated at first, its validity gradually became overwhelming as its application accounted for many discrepancies between observed phenomena and classical theory. German physicist Max Planck publishes his groundbreaking study of the effect of radiation on “blackbody”, and the quantum theory of modern physics is born. Scientists, such as Einstein, Bohr, de Broglie, Schrodinger and Dirac, advanced Planck’s theory and made possible the development of the quantum theory that maintains that energy is both matter and a wave. Quantum mechanics takes a probabilistic view of nature, sharply contrasting with classical mechanics, in which all precise properties of objects are, in principle, calculable. Who is Planck? https://www.khanacademy.org/science/physics/thermodynamics/temp-kinetic- theory-ideal-gas-law/a/what-is-the-maxwell-boltzmann-distribution https://www.khanacademy.org/science/physics/thermodynamics/temp-kinetic- theory-ideal-gas-law/v/maxwell-boltzmann-distribution Maxwell-Boltzmann Distribution Video Text Blackbody Observations - https://youtu.be/FRd28CLvMQM Rayleigh Jeans Law - https://youtu.be/kHz6zbDqifQ Planck Energy Distribution - https://youtu.be/tAZYKNKkxs4 Summary - https://youtu.be/KR8SmZ5fGIg Teaching videos on black body radiation • Oscillators present in blackbody absorb and emit light in packets. The absorption and emission energies are quantized, and it can’t take all values. • The energy of photons absorbed and emitted can be expressed in the form: E = hn • What is an oscillator? Nobel Prize in Physics 1922 Nobel Prize in Physics 1918 • Absorption and emission in an atom are due to electronic transitions. • Oscillators are in a way electrons in materials. Bohr Planck Further support for quantization of energy • Bohr interpreted some experimental results that had been known for nearly thirty years. • It had been known for most of the nineteenth century that elements, when vaporized by heat and made to glow, displayed “line spectra” of this sort. The so-called Balmer series of hydrogen lines had been found in 1885. • Remarkably, Bohr was able to derive his formula from his quantum model of the hydrogen atom. Bohr Model The studies of light revealed that electrons can have only specific energies and only certain allowable orbits to represent those energies. In other words, energy is “quantized” in atoms. It can only be absorbed and released in specific amounts, not any amount! Before the quantum model of the atom. Rutherford Model The quantum model of the atom rungs of a ladder. Bohr Model The Bohr Planetary Model • Bohr suggested that the electrons in an atom orbit the positively-charged nucleus, in a similar way to planets orbiting the Sun. • Bohr made the bold assumption that the orbital angular momentum of the electron is quantized. • Since v is perpendicular to r, the orbital angular momentum is just given by L = mvr. • Bohr suggested that this is quantized, so that: n nh mvr == p2 Nucleus E3 E2 E1 E5 E4 Bohr Model of H Atom Atomic orbitals replace oscillators Light is emitted when an electron jumps from a higher orbit to a lower orbit and is absorbed when it jumps from a lower to higher orbit. The energy and frequency of light emitted or absorbed is given by the difference between the two orbit energies, e.g., E(photon) = E2 - E1 (Energy difference) Planck’s oscillator emission and absorption are in fact electronic transitions between orbitals of fixed energies. Transition energies are quantized. Light is emitted when an electron jumps from a higher orbit to a lower orbit and is absorbed when it jumps from a lower to higher orbit. The energy and frequency of light emitted or absorbed is given by the difference between the two orbit energies, e.g., E(photon) = E2 - E1 (Energy difference) Electronic transitions are quantized Atomic orbitals replace oscillators Absorption Emission The basis of all photochemistry and spectroscopy! Light is a wave Light is electromagnetic Light absorption and emission are quantized Light induces electronic transitions (oscillators are in fact electrons) What we know thus far Photoelectric Effect Light Electrons Metal’s electrons near the surface are ejected upon light absorption In 1887 H. Hertz of Germany was the first person to detect the photoelectric effect. He knew something negatively charged species was coming out. At that time electrons were unknown. So not known what was being ejected. In 1899, J. J. Thompson of England, demonstrated that ultraviolet light hitting a metal surface caused the ejection of electrons In 1905 Einstein, then a young patent clerk in Switzerland, explained the phenomenon. In 1921 Einstein received the Nobel Prize after Robert Millikan confirmed the work. Photo-Electric Effect J. J. Thomson (1856-1940) Nobel Prize 1906 In 1889 Thompson notices that UV light releases electrons from cathode. Philipp Lenard (1862-1947) Nobel Prize, 1905 From 1892-1894 worked as Hertz’s assistant. In 1902 Established the importance of the wavelength of light in releasing electrons from a surface and showed that the velocity of released electrons is independent of the intensity of the light. Heinrich Hertz (1857-1894) In 1887 Hertz notices that the charged objects illuminated with UV light loses charge. A. Einstein (1879-1955) Nobel Prize 1921 In 1905 theoretically explained the photoelectric phenomenon on the basis light is a particle. R. A. Millikan (1868 – 1953) Nobel Prize 1923 In 1914 experimentally confirmed Einstein’s predictions and measured the value of h. • Anti-semitic • Supporter and advisor to Hitler • Proponent of Nazi ideology • Did not believe in Einstein’s works, had open quarrels with him Philipp Lenard R. A. Millikan • Data selection, ethical issues • Racism issues Classical Theory – Light is a wave - Predictions metal light electrons e- e- eLow Intensity - Small Wave Light wave “hits” electron gently. Electrons come out – low speed. High Intensity - Big Wave Light wave “hits” electron hard. Electrons come out – high speed. Only intensity matters Analysis of Photoelectric Effect Based on Classical Mechanics-1 •Dependence of photoelectron kinetic energy on light intensity Classical Predictions • Wavelength is not critical, intensity alone matters. Higher intensity at longer wavelength should work. • At low light intensities, measurable time interval should pass between the instant the light is turned on and the time an electron is ejected from the metal (time delay). Energy needs to be built up. • Experimental Results • Electrons are emitted almost instantaneously, even at very low light intensities if the wavelength is correct. Dependence of photoelectron kinetic energy on light frequency • Classical Predictions • There should be no relationship between the frequency of the light and the electron’s kinetic energy. • The kinetic energy should be related to the intensity of the light. • Experimental Results • The maximum kinetic energy of the photoelectrons increases with increasing light frequency. • The maximum kinetic energy of the photoelectrons is independent of the intensity. If the wavelength is below the stopping potential intensity will play no role. If the If the wavelength is above the stopping potential more number of electrons would come with the same kinetic energy. Analysis of Photoelectric Effect Based on Classical Mechanics-2 Einstein’s Model of Light: Photon Torpedoes Light travels as a wave and hits like a particle. E = mc2. Light energy comes in packets. Each photon has an energy of E = hn Light itself is quantized, not only absorptions and emissions. Needed energy must be provided in a single photon. For example, if 100 kcal/mole is needed for an action, 5x20 kcal/mole will not work. One photon of 100 kcal/mole is needed. One photon interacts only with single electron Light has a wave-particle duality behavior. Einstein, 1905 "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, localized in space, which move without being divided and which can be absorbed or emitted only as a whole". “---- the production of cathode rays (electrons) by light can be conceived in the following way: The body’s surface layer is penetrated by energy quanta whose energy is converted at least partially into kinetic energy of the electrons. The simplest conception is that a light quantum transfers its energy to a single electron----” Einstein’s View on Photoelectric Effect Annalen der Physik, 1905, 26 years old, six weeks before submission of his Ph. D. thesis Albert Einstein Nobel Prize, 1921 Not only absorption and emission are quantized (Planck), but light itself is quantized Three Three Energy Out Metal Surface Since photons have particle-like properties, they should have mass. The (relativistic) mass of photons can be calculated from Einstein’s equation for special relativity. If photon is particle, does it have mass ? Photon has ZERO mass at rest; it has mass when it is moving at the speed of light, C. (relativistic mass) = p/c = Momentum/Cm Photon momentum is small, since p = h/λ and h is very small. It is for this reason that we do not ordinarily observe photon momentum. Our mirrors do not recoil when light reflects from them. p = m × v where m = mass of the object and v = velocity of the object. m p × v where v = c (speed of light) l h p = If photon is particle, does it have momentum ? h = 6.626 070 15 x 10-34 J Hz-1 ! ≥ !# ! < !# light e- (Photoelectron) Metal light e- (Photoelectron) Metal Photoelectrons are only ejected for frequency of light greater or equal to a particular frequency (depends on the metal) If the frequency is less than that threshold frequency then no photoelectron is ejected. Einstein’s prediction about the energy of released electron Einstein predicted a linear relationship between the kinetic energy of the released electron and the frequency of irradiation (𝒉n) with the slope being equal to 𝒉 and the intercept being the work function of the metal (𝒉no) 𝑲𝑬 𝒎𝒂𝒙= 𝒉n − 𝒉no 𝒉n ≥ 𝒉no Slope (m) for all metals = 6.626 × 10-34 Js = h " $% &'(ℎ* "+(-.) Intercept (c)= -(6.626 × 10-34× "0) J y = mx + c K.E. of e- "+(0) "+(12) −"+(-.)ℎ −"+(0)ℎ −"+(12)ℎ 1. Slope is same for all the metal and is equal to Planck’s constant (h) 2. KE = h 𝑣 - h 𝑣! Incident energy Threshold energy (Work function) 4. Light is made up of energy “packets” called photons, light energy is quantized 3. The surface takes only h 𝑣! and that needs to be delivered in one packet. Einstein analyzed plots of K.E. of photoelectrons as a function of frequency for different metals (1905) KE = h 𝑣 - h 𝑣! Other Scientists’ Reactions to Einstein’s Explanation of the Photo-Electric Effect The original data used to generate the idea were actuallyless clear than they implied. Even though it proved his own theory, Planck himself was skeptical. R. S. Millikan spent ten years trying to disprove Planck’s proposal, but finally grudgingly published data supporting it in 1916. Einstein finally won the Nobel Prize for this work in 1921 for photoelectric effect and Millikan won the Nobel Prize in 1923 for it (and oil-drop experiment). Most physicists believed that light is a wave and did not want to go back to particle idea. Photon Particle Collisions - The Compton Effect In 1922 Arthur Compton was able to bounce an X-ray photon off an electron. The result was an electron with more kinetic energy than it started with, and an X-ray with less energy than it started with. A photon can actually interact with a particle! A photon has momentum!! A. H. Compton (1892-1962) Nobel Prize, 1927 Professor, Uni Chicago President, MIT Washington Uni Compton found that if you treat the photons as if they were particles of zero mass, with energy and momentum . the collision behaves just as if it were two billiard balls colliding ! (with total momentum always conserved) E = hc/ p = h/ Compton showed that when X-rays impinged on matter, the scattered X-ray did not have the same wavelength ! ✓i f incident photon target electron at rest recoil electron scattered photon Solar sail Solar sailing is a revolutionary way of propelling a spacecraft through space. Light is made up of particles called photons. Photons don’t have any mass, but as they travel through space they do have momentum. When light hits a solar sail — which has a bright, mirror-like surface — the photons in that light bounce off the sail. As the photons hit the sail their momentum is transferred to it, giving it a small push. As they bounce off the sail, the photons give it another small push. Both pushes are very slight, but in the vacuum of space where there is nothing to slow down the sail, each push changes the sail’s speed. https://www.youtube.com/watch?v =Ndx_6J4uo2M https://www.planetary.org/articles/w hat-is-solar-sailing Light is always both a Wave and a Particle ! -Light behaves like a wave when it propagates through space -And as a particle when it interacts with matter Muhammad Ali • Historically, light was thought of as a stream of particles until Young’s experiments proved light has wave-like properties. • Planck was working with the wave notion of light when he related the energy of blackbody radiation to the frequency of emitted light. E = hν • Einstein began to consider the particle viewpoint again when trying to explain the photoelectric effect. E = mc2 Max Planck and Albert Einstein Berlin, June 1929 Interference: The Double Slit Experiment particle? wave? Double Slit Experiment • What happens if we close one or the other slits? • What happens if we send one photon at a time towards the two slits? • What happens if we monitor which slit the single photon entered? The double slit experiment with single photon (strange results) Double Slit Experiment What happens if we close one of the slits? No interference pattern. Just the diffraction pattern from the single open slit. The Double Slit Experiment with Single Photon Very few photon Many photons x • What happens if we send one photon at a time towards the two slits? • We see individual “hits” corresponding to each photon • But as the photons arrive one by one over time, they build up an interference pattern Double Slit Experiment • What happens if we monitor which slit the single photon entered? Detector on Detector off No interference pattern Interference pattern • The pattern on the screen is an interference pattern characteristic of waves • So light is a wave, not particulate • But repeat the experiment one photon at a time • Over time, the photons only land on the interference peaks, not in the troughs • consider the fact that they also pile up in the middle! • pure ballistic particles would land in one of two spots The double slit experiment with single photon (strange results) https://www.youtube.com/watch?v=O81Cilon10M2. Niel Johnson, Double slit The double slit experiment with single photon (strange results) https://www.youtube.com/watch?v=Ms-CVF540fo3. Neil deGrasse Tyson, Nova https://www.youtube.com/watch?v=A9tKncAdlHQ1. Jim Al-Khalili https://www.youtube.com/watch?v=e5_V78SWGF04. Light-Summary-1 https://www.youtube.com/watch?v=FlIrgE5T_g05. Electron-Summary-2 Being in Two Places at the Same Time… This is a famous statement by the great 20thcentury physicist, Paul Dirac. It means that each photon acts like a wave (that is, extends over space) at the slits and so interferes with itself to produce the two-slit pattern. Paul Dirac (1902-1984) Nobel Prize, 1933This is true of all wave/particles (wavicles). Photons don’t interfere with each other. They interfere with themselves. Light is made up of photons. Light is measured in terms of Einstein. One Einstein is the energy in one mole (6.022 x 1023) of photons. Energy of one E depends on the frequency of photon. Liquid water is made up of molecules. Amount is measure in terms of mole (M). One mole contains 6.022×1023 molecules (Avogadro's number). Weight of one M depends on the weight of the molecule. Photon “Thus light is something like raindrops-each little lump of light is called a photon-and if the light is all one color, all the "raindrops" are the same size.” Richard P. Feynman Nobel Prize, 1965 “All the fifty years of conscious brooding have brought me no closer to the answer to the question, 'What are light quanta?' Of course today every rascal thinks he knows the answer, but he is deluding himself.” “As far as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality.” Albert Einstein “For the rest of my life I will reflect on what light is.” (1917)