Transcript X-rays - Skulls in the Stars

Forgotten milestones
in the history of optics
Greg Gbur
Department of Physics and Optical Science
UNC Charlotte
Introduction
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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
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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
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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
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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 Xrays have polarization in 1905
Modern: Leonard Mandel shows multi-photon
interference in 1963
Prehistory of optics
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Earliest “scientific” studies of light could be
attributed to Aristotle, Ptolemy, Euclid:
Aristotle (384-322 B.C.E.)
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Euclid (c. 300 B.C.E.)
Ptolemy (90-168 C.E.)
Light and vision were concepts essentially
independent (but intertwined)
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 alHaytham, 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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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 Xray 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
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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
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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
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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…
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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.