Forgotten milestones in the history of optics

Forgotten milestones in the history of optics Greg Gbur Department of Physics and Optical Science UNC Charlotte

Introduction • History is important! • A proper study of historical experiments can give crucial context, and understanding • Many important and enlightening experiments have been “forgotten” by science • An understanding of such experiments can provide inspiration and a better understanding of the philosophy of science

Periods of optical history • Prehistory: initial studies of optics and vision • Particle: light treated as a stream of particles • Wave: light treated as a continuous wave • Quantum: light has wave/particle duality • Modern: light even weirder than we imagined!

Periods of optical history • Prehistory: Aristotle, Ptolemy, Ibn al-Haytham • Particle: Newton published Optiks in 1704 • Wave: Young published double slit experiment in 1803 • Quantum: Einstein published photoelectric effect in 1905 • Modern: Maiman builds first laser in 1960

Periods of optical history • Prehistory: Ibn al-Haytham writes Book of Optics between 1011-1021 C.E. • Particle/Wave: François Arago studies stellar aberration in 1810 • Wave/Quantum: Charles Barkla shows that X-rays have polarization in 1905 • Modern: Leonard Mandel shows multi-photon interference in 1963

Prehistory of optics • Earliest “scientific” studies of light could be attributed to Aristotle, Ptolemy, Euclid: • Light and vision were concepts essentially independent (but intertwined) Aristotle (384-322 B.C.E.) Euclid (c. 300 B.C.E.) Ptolemy (90-168 C.E.)

Models of vision Adapted from Bradley Steffens, Ibn al-Haytham, First Scientist (Morgan Reynolds, Greensboro, NC, 2007), as is much of the discussion of al-Haytham. A great HS-level introduction to Ibn al-Haytham, and the only popular biography I know of.

Ibn al-Haytham (965-1039 C.E.) • born in Basra (Iraq), devoted his early life to theology, but grew frustrated with sectarian arguments • Discovered the works of Aristotle as a young man, and devoted his life to the study of the physical world • Studied, and eventually commented on, works of Aristotle, Euclid, Archimedes, Ptolemy (from Iraqi 10 dinar note) • Appointed a vizier in the Basra government, but was dismissed from the job by either feigned or actual mental illness • Wrote possibly more than 200 works, with some 50 still surviving in some form • Provided the first description of a “scientific method” • Studied optics, astronomy, geometry, mechanics, water clocks, medicine, anatomy, business arithmetic, even civil engineering!

Islamic Renaissance • Muslim scholars carefully studied the works of ancient Greeks (Aristotle, Plato, Archimedes, Euclid, Ptolemy) and many translations existed • caliph Abu Jafar al-Ma’mun ibn Harun of Iraq founded the Bait-ul-Hikmat (“House of Wisdom”) around 813 C.E., in Baghdad • In 825, Muhammed ibn Musa al-Khwarizmi adopted Arabic numerals from Hindu mathematicians • al-Khwarizmi also introduced algebra to the Muslim world (al-jabr)

Ibn al-Haytham in Cairo • Called to Cairo around 1010 C.E. by “The Mad Caliph” al-Hakim, to attempt to dam the Nile! • Project was determined to be infeasible • Seems to have been given a government post nevertheless (though accounts vary) Aswan high dam • Soon after, mental illness came back – or he began faking it in order to get out of government duties! (like modern-day jury duty?) • Was placed under house arrest in Cairo for a period of ten years, deprived of his belongings • It seems that during this period he developed his Book of Optics!

al-Haytham’s Book of Optics • Seven-volume book on vision, the anatomy of the eye, light propagation, reflection, and refraction • Introduces rectilinear propagation of light: light travels in straight lines from object to eye • “Sight does not perceive any visible object unless there exists in the object some light, which the object possesses of itself or which radiates upon it from another object” • First to make the (seemingly obvious) connection between light and vision • First to observe that the brain is the center of vision, not the eye • Introduced the distinction between primary and secondary sources • Performed the first non-trivial demonstration of the camera obscura

Camera obscura Using geometrical optics, we can demonstrate that light passing through a small pinhole into a darkened room forms a “reversed” image of the object: Naturalists prior to al-Haytham had observed this type of effect via, for instance, sunlight traveling through gaps in the leaves, but none apparently had studied the phenomena systematically

Ibn al-Haytham’s Camera obscura Ibn al-Haytham used multiple light sources to demonstrate that light followed straight line paths through the holes: By screening one light source or another, was able to demonstrate that the “image” was inverted on passing through the hole!

Ibn al-Haytham conclusions • Experiment was not done to demonstrate imaging, but rather the non-interaction of light rays with one another: • “all the lights that appear in the dark place have reached it through the aperture alone… therefore the lights of all those lamps have come together at the aperture, then separated after passing through it. Thus, if lights blended in the atmosphere, the lights of the lamps meeting at the aperture would have mixed in the air at the aperture… and they would have come out so mingled together that they would not be subsequently distinguishable. We do not, however, find the matter to be so; rather the lights are found to come out separately, each being the opposite the lamp from which it has arrived.”

al-Haytham’s influence • al-Haytham’s Book of Optics remained one of the most influential optics books throughout the prehistory period • translated into Latin; al-Haytham Latinized to “Alhazen” or “Alhacen” • influenced significant medieval optical researchers such as Roger Bacon (1214-1294)

Corpuscular era of light • By late 1600s, basics of geometrical optics had been established (Snell’s law in 1621, Fermat’s principle of least time in 1662) • In 1690, Christiaan Huygens published Traité de la Lumière, suggesting light is a wave phenomenon • Newton’s 1704 Optiks, however, firmly cemented the corpuscular (particle) theory of light for 100 years

Transition to the wave era • In 1803, Thomas Young published his famous double slit experiment demonstrating the wave nature of light; however, the result was not immediately recognized • An explanation of diffraction was proposed as the subject of the 1818 Paris Academy prize question • Augustin Jean Fresnel explained diffraction based on a wave theory of light • Poisson argued against it, stating that the theory would lead to a bright spot behind an opaque disk; Arago experimentally found the spot! (Arago spot)

François Arago (1786-1853) • A French physicist, mathematician, astronomer, politician – and unwilling adventurer! • Made fundamental contributions to optics: the Arago spot, the Fresnel-Arago laws, and the stellar aberrations to be mentioned, among others • After 1830s, was an active “liberal republican” in French politics, and his influence and guidance helped spur many scientific discoveries • In 1806, went to Spain to perform meridional measurements; in June 1808, he was accused as a spy and imprisoned in a fortress; in July 1808 he escaped in a fishing boat, reaching Algiers in August. Mid-August, sailing to France, his ship was captured by a Spanish corsair, and he was imprisoned in Spain until November! Freed, his next trip to Marseilles was blown back by a bad wind to the coast of Africa, at which point he took a six month trip back to Algiers on land. Finally sailing to Marseilles, he was quarantined for some time!

The speed of light • Measurements of the speed of light had first been made by Römer in 1676: Essentially the Doppler effect!

Stellar aberration The combination of the finite speed of light and the motion of the earth leads to stellar aberration, a phenomenon in which starlight appears to come from different directions at different times of year (first observed in 1725 by James Bradley):

Stellar aberration • Aberration angle: tan = v/c • Can in principle use stellar aberration to measure the speed of light • Researchers of the time were interested in measuring variations in the speed of light • Heavier stars were expected to produce slower light in the corpuscular theory • Stellar aberration alone not precise enough to measure difference

Newtonian view of refraction According to Newtonian theory of refraction, light particles refract because they speed up in matter; i.e., speed c becomes speed nc: To reproduce Snell’s law (with n2 = n), must have:

Arago’s experiment (1810) Arago realized that light traveling at different initial speeds should be refracted at different angles: same direction of incidence refraction of light 1 refraction of light 2 refraction angles different!

Results • Arago found that the light from every star is refracted by the same amount! • “This result seems to be, with the first aspect, in manifest contradiction with the Newtonian theory of the refraction, since a real inequality in the speed of the rays however does not cause any inequality in the deviations which they test. It even seems that one can return of it reason only by supposing that the luminous elements emit rays with all kinds speeds, provided that it is also admitted that these rays are visible only when their speeds lie between given limits. On this assumption, indeed, the visibility of the rays will depend their relative speeds, and, as these same speeds determine the quantity of the refraction, the visible rays will be always also refracted.”

Conclusions and impact • Newton’s particle theory of light completely failed to explain Arago’s experiment: a wave theory of light seemed the only possibility • In 1818, Fresnel suggested that the aether, the hypothetical medium in which light travels, is partially dragged along with a material medium • Result led Arago to embrace the wave theory of light, and also led to widespread belief in the aether! • (True explanation of Arago’s results in special relativity) • A failed experiment, based on incorrect theories of light propagation, interpreted incorrectly by Fresnel, but which helped convince people of the (correct) wavelike properties of light!

The wave era of light • Fresnel and Arago (1816) showed that orthogonal polarizations would not interfere • Young (1817) interpreted light as a transverse wave • Ørsted, Ampère (1820) and Faraday (1831) showed that electricity and magnetism are related • Maxwell (1864) laid the theoretical foundations for light as an electromagnetic wave • Hertz (1887) experimentally demonstrated electromagnetic waves

X-rays • In 1895, Wilhelm Conrad Röntgen discovered a mysterious new form of radiation, by accident, dubbed “X-rays” • After two short weeks of experiments, the first X-ray photograph was produced of the human body, using his wife Anna as a test subject Rays produced when high-energy electrons collide with an anticathode in a “cathode ray tube”; this one is a “Cossor tube”

Are X-rays electromagnetic waves? Physical origin of X-rays was not immediately clear. Were they a new form of particle? A new form of wave? Or another manifestation of electromagnetic waves? Three properties of X-rays seemed very unlike light and other E/M radiation: • X-rays did not seem to be refracted when entering a material surface • X-rays are reflected diffusely at a surface, instead of being reflected in a single direction (specular reflection) • X-rays did not seem to experience diffraction

Charles Glover Barkla (1877-1944) • British physicist who worked as a professor of natural philosophy at the University of Edinburgh from 1917 until his death • Ph.D. advisors were J.J. Thomson, discoverer of the electron, and O. Lodge, a key developer of “wireless telegraphy” • Worked primarily in X-ray scattering, X-ray spectroscopy, and the excitation of “secondary” X-rays • Won the Nobel Prize in 1917 for “his discovery of the characteristic X-radiation of the elements.”

Wilberforce’s idea (I) Polarization would be a good indication of the electromagnetic nature of X-rays; however, ordinary methods of polarizing light do not work for X-rays: they shoot right through polarizers, and because they don’t specularly reflect Brewster’s angle doesn’t work. Researchers knew that X-rays passing through gas scatter and produce “secondary” X-rays:

Wilberforce’s idea (II) Professor Wilberforce suggested to Barkla that one could use the secondary radiation as a polarized source, and scattering the secondary radiation, produce a tertiary beam of radiation, which should have a dipole behavior: Unfortunately, secondary radiation is weak: tertiary radiation is negligible!

Barkla’s experiment (1905) Barkla realized, however, that polarized X-rays must be produced right at the anticathode: An appropriately-collimated beam of radiation from the anticathode could be scattered from a gas, and the secondary radiation would have polarization properties!

Barkla’s experiment (II) Rotation of the bulb should result in the secondary radiation appearing in the vertical position, for horizontal rays, or the horizontal position, for vertical rays

Barkla’s results “As the bulb was rotated round the axis of the primary beam there was, of course, no change in the intensity of primary radiation in that direction. There was, however, a considerable change in the intensity of secondary radiation in both the horizontal and vertical directions, one reaching a maximum when the other attained a minimum. By turning the bulb through a right angle the electroscope which had previously indicated a maximum of intensity indicated a minimum, and vice versa. The position of the bulb when the vertical secondary beam attained a maximum of intensity and the horizontal secondary beam a minimum was that in which the kathode stream was horizontal, the maximum and minimum being reversed when the kathode stream was vertical. By turning the bulb through another right angle, so that the kathode stream was again horizontal but in the opposite direction to that in the other horizontal position, the maximum and minimum were attained as before.”

Quantum era of light • The same year of Barkla’s publication (1905), Einstein developed special relativity and explained the photoelectric effect by (re)postulating the particle nature of light • Quantum mechanics was developed rapidly to explain atomic structure and the nature of the light/matter interaction • Detailed studies of the behavior of light particles (photons) was somewhat hindered by the lack of a “quality” light source

Paul Dirac’s (in)famous statement In his 1930 text Principles of Quantum Mechanics, the brilliant scientist Paul Dirac made the following statement: “Some time before the discovery of quantum mechanics people realized that the connexion between light waves and photons must be of a statistical character. What they did not clearly realize, however, was that the wave function gives information about the probability of one photon being in a particular place and not the probable number of photons in that place. The importance of the distinction can be made clear in the following way. Suppose we have a beam of light consisting of a large number of photons split up into two components of equal intensity. On the assumption that the intensity of a beam is connected with the probable number of photons in it, we should have half the total number of photons going into each component. If the two components are now made to interfere, we should require a photon in one component to be able to interfere with one in the other. Sometimes these two photons would have to annihilate one another and other times they would have to produce four photons. This would contradict the conservation of energy. The new theory, which connects the wave function with probabilities for one photon, gets over the difficulty by making each photon go partly into each of the two components. Each photon then interferes only with itself. Interference between two different photons never occurs.”

Quantum to modern era • In 1917, Einstein established the theoretical foundations of stimulated emission • In 1953, Charles H. Townes and students produced the first microwave amplifier based on this principle (the first MASER was built in Russia at a similar time, by Basov and Prokhorov) • Theodore H. Maiman produced the first working LASER in 1960 • Basov, Prokhorov and Townes shared the 1964 Nobel Prize in Physics

Leonard Mandel (1927-2001) • Born in Berlin, Germany • Earned his Ph.D. in nuclear physics from Birkbeck College, University of London, in 1951 • One of the pioneers of the field of quantum optics, which has led to such speculative ideas as quantum computing, quantum cryptography, and quantum teleportation (and he had a hand in all of them) and tested the foundations of quantum mechanics itself • Became a faculty member at the Institute of Optics at the University of Rochester in 1964, at the invitation of colleague Professor Emil Wolf

Interference of independent beams • Interference requires, in essence, that the wave fields being interfered have a definite phase relationship with respect to each other • Two independent lasers will fluctuate independently of one another and on average will produce no discernable interference pattern • Note the clause “on average”; is the absence of interference just an artifact of the averaging process, or a true manifestation of Dirac’s statement, “Interference between two different photons never occurs”?

Interference of independent beams Classically, two quasi-monochromatic waves will stay in phase for a finite period of time; during that time, it should be possible to see an interference pattern between them:

Magyar and Mandel’s experiment (1963) “Two light beams from two independent ruby masers are aligned with the help of two adjustable 45º mirrors and superposed on the photocathode of an electronically gated image tube. The tube is magnetically focused and the image produced on the output fluorescent screen is photographed.”

M&M results Fringes were observed, as can be seen both in the photocathode image (left) and the microphotometer tracing on the right: So it would seem that different photons can interfere with one another, violating Dirac’s original statement… or can they?

The quantum plot thickens… • In 1967, L. Mandel and R.L. Pfleegor would repeat the experiment with very low intensity light sources • With high probability, only one photon is present at the detector at any time • They still found an interference pattern! • “Surprising as it might seem, the statement of Dirac quoted in the introduction appears to be as appropriate in the context of this experiment as under the more usual conditions of interferometry.” • (I would say that Dirac’s statement is appropriate, but not a terribly useful one in the context of this experiment)

A “Meta” era of optics? Recent discoveries have shifted the focus of optics from, “What is the behavior of light?” to, “How can we make light behave how we want it to?” • T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, and P.A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature 391 (1998), 667. • J.B. Pendry, “Negative Refraction Makes a Perfect Lens,” Phys. Rev. Lett. 85 (2000), 3966. • U. Leonhardt, “Optical Conformal Mapping,” Science 312 (2006), 1777. • J. B. Pendry, D. Schurig, D. R. Smith, “Controlling electromagnetic fields,” Science 312 (2006), 1780.

Investigating the history of optics Pretty much any historical paper, from 1600s through the 1930s, can be found freely available on Google books and through other sources! • Bradley Steffens, Ibn al-Haytham, First Scientist (Morgan Reynolds, Greensboro, NC, 2007) • François Arago, Œuvres Complètes, Tome 7, Volume 4 (1858), p. 548-568. • Charles Barkla, “Polarisation in Röntgen rays,” Nature 69 (1904), 463. • Charles Barkla, “Polarized Röntgen radiation,” Phil. Trans. Roy. Soc. Lond. A 204 (1905), 467. • G. Magyar and L. Mandel, “Interference fringes produced by superposition of two independent maser light beams,” Nature 198 (1963), 255. • R.Pfleegor and L. Mandel, “Interference of independent photon beams,” Phys. Rev. 159 (1967), 1084.

Forgotten milestones in the history of optics