1. History of the Speed of Light ( c )
History of c Askey RET summer 2002
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History of the Speed of Light ( c )
Jennifer Deaton and
Tina Patrick
Fall 1996
Revised by David Askey
Summer RET 2002
History_of_c-2002_Askey

2. History of c Askey RET summer 2002
Introduction
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The speed of light is a very important fundamental constant known with great precision today due to the contribution of many scientists. Up until the late 1600's, light was thought to propagate instantaneously through the ether, which was the hypothetical massless medium distributed throughout the universe. Galileo was one of the first to question the infinite velocity of light, and his efforts began what was to become a long list of many more experiments, each improving the accuracy of c.
This was the original slide, I will rewrite this
Add a watermark behind it having something to do with light
History_of_c-2002_Askey

3. Is the Speed of Light Infinite?
History of c Askey RET summer 2002
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Is the Speed of Light Infinite?
Galileo’s Simplicio, states the Aristotelian (and Descartes) position,
“Everyday experience shows that the propagation of light is instantaneous; for when we see a piece of artillery fired at great distance, the flash reaches our eyes without lapse of time; but the sound reaches the ear only after a noticeable interval.”
Galileo in Two New Sciences, published in Leyden in 1638, proposed that the question might be settled in true scientific fashion by an experiment over a number of miles using lanterns, telescopes, and shutters.
This slide is from :
Early Ideas about Light Propagation
In 1600 A.D., Kepler maintained the majority view that light speed was instantaneous, since space could offer no resistance to its motion
As we shall soon see, attempts to measure the speed of light played an important part in the development of the theory of special relativity, and, indeed, the speed of light is central to the theory.
The first recorded discussion of the speed of light (I think) is in Aristotle, where he quotes Empedocles as saying the light from the sun must take some time to reach the earth, but Aristotle himself apparently disagrees, and even Descartes thought that light traveled instantaneously. Galileo, unfairly as usual, in Two New Sciences (page 42) has Simplicio stating the Aristotelian position,
SIMP. Everyday experience shows that the propagation of light is instantaneous; for when we see a piece of artillery fired at great distance, the flash reaches our eyes without lapse of time; but the sound reaches the ear only after a noticeable interval.
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4. History of c Askey RET summer 2002
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1667 Lantern Experiment
The Accademia del Cimento of Florence took Galileo’s suggestion and made the first attempt to actually measure the velocity of light.
Two people, A and B, with covered lanterns went to the tops of hills about 1 mile apart.
First A uncovers his lantern. As soon as B sees A's light, he uncovers his own lantern.
Measure the time from when A uncovers his lantern until A sees B's light, then divide this time by twice the distance between the hill tops.
Therefore, the speed of light would theoretically be c = (2D)/t.
Human reaction times are approx. 0.2 sec and therefore, too slow to determine c with any accuracy.
Proved speed of light was finite and showed that light travels at least 10x faster than sound. A
Approx one mile B
The Accademia del Cimento of Florence reported in 1667 that such an experiment over a distance of one mile was tried, "without any observable delay” from
Find out how far away they were. How fast is human reaction time (. Approx .2 sec) light and shutter. Look up more on this.
From this one can certainly deduce that light travels at least ten times faster than sound.
In 1667, Galileo suggested a method for actually measuring the speed of
light. The method was to take two people A and B, covered lanterns to
the tops of hills that are separeted by a distance of about a mile. First
A uncovers her latern. As soon as B sees A's light, she uncovers her own
lantern. By measuring the time from when A uncovers her lantern until A
sees B's light, then dividing this time by time by twice the distance
between the hill tops, the speed of light can be determined. Galileo was
able to determine only that the speed of light was far greater than could
be measured using his procedure. Althougt Galileo was unable to provide
even an approximate value for the speed of light, his experiment set the
stage for later attempts.
History_of_c-2002_Askey

5. Longitude and Jupiter’s Moons
History of c Askey RET summer 2002
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Longitude and Jupiter’s Moons
Thousands of men were lost at sea because there was no accurate way of determining longitude at sea.
Galileo proposed using an eclipse of one of Jupiter’s moons to determine the difference in longitude between two places.
Olaf Roemer took up the task of using Jupiter’s moon’s to determine longitude.
I want to include this slide about Galileo’s Jupiter moon solution to the longitude problem and how it attracted the attention of Roemer
I found a couple of good web sites for pictures.
astronomy.swin.edu.au/~pbourke/geometry/sphere/
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6. 1676: First “Hard Evidence” For the Finite Speed of Light
History of c Askey RET summer 2002
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Olaf Roemer noticed variations in the eclipse times of Io, the innermost moon of Jupiter.
When the Earth moved away from Jupiter, the moon appeared to stay behind the planet 22 minutes longer than when the Earth was moving towards Jupiter.
He used the equation: c = (d1 - d2)/(t1 - t2)
t2 = time of eclipse when the Earth is moving toward Jupiter
t1 = time of eclipse when the Earth is moving away
d2 = distance the Earth moves during t2.
d1 = distance the Earth travels during time t1,
Roemer determined that c = 2.1 x 108 m/s.
One third to slow because he was using inaccurate information on the radius of the Earth's orbit Io
Eclipse lasts longer than it should
Ole Roemer ( ) was a Danish astronomer.
At the time only 4 of Jupiter’s moons had been discovered and their periods were known.
Double check this equation
Io had a period of 42.5 hours and its orbit, the orbit of Jupiter, and Earth’s orbit all lie in approximately the same plane
Io, therefore, goes into eclipse behind Jupiter every 42.5 hours as seen from the Earth.
Think about changing the drawing to the one in the longitude book. Show it to Matt.
The first successfull measurement of the velocity of light was provided by
the Danish astronomer Olaf Romer in He based in measurement on
observations of the eclipses of one of the moons of Jupiter. As this moon
orbits Jupiter, there is a period of time when Jupiter lies between it and
the earth, and blocks it from view. Romer noticed that the duration of
these eclipses was shorter when the Earth was moving toward Jupiter than
when the Earth was moving away. He correctlly interpreted this phenomena
as resulting form the finite speed of light.
Geometrically the moon is always behind Jupiter for the same period of
time drign each eclipse. Suppose, however, that the Earth is moving away
from Jupiter. An astronomer on Earth catches his last glimpse of the
moon, not at the instant the moon moves behind Jupiter, but only after the
last bit of unblocked light form the moon reaches his eyes. There is a
similar delay as the moon moves out form behind Jupiter but, since the
Earth has moved father away, the light must now travel a longer distance
to reach the astronomer. The astronomer, therefore sees an eclipse that
,lasts longer the actual geometrical eclipse. Similarly, when the Earth is
moving toward Jupiter, the astronomer sees and eclipse that lasts a
shorter interval of time.
From observations of these eclipses over many years, Romer calculated
the speed of light to be 2.1*10^8 m/sec. This value is approximately 1/3
too slow due to inaccurate knowledge at at that time of the distances
involved. Nevertheless, Romer's method provided clear evidence that the
velocity of light was not infinite, and gave a reasonable estimate of its
true value-not bad for 1675.
Measuring the Speed of Light with Jupiter's Moons
The first real measurement of the speed of light came about half a century later, in 1676, by a Danish astronomer, Ole Römer, working at the Paris Observatory. He had made a systematic study of Io, one of the moons of Jupiter, which was eclipsed by Jupiter at regular intervals, as Io went around Jupiter in a circular orbit at a steady rate. Actually, Römer found, for several months the eclipses lagged more and more behind the expected time, until they were running about eight minutes late, then they began to pick up again, and in fact after about six months were running eight minutes early. The cycle then repeated itself. Römerrealized the significance of the time involved-just over one year. This time period had nothing to do with Io, but was the time between successive closest approaches of earth in its orbit to Jupiter. The eclipses were furthest behind the predicted times when the earth was furthest from Jupiter.
The natural explanation was that the light from Io (actually reflected sunlight, of course) took time to reach the earth, and took the longest time when the earth was furthest away. From his observations, Römer concluded that light took about twenty-two minutes to cross the earth's orbit. This was something of an overestimate, and a few years later Newton wrote in the Principia (Book I, section XIV): "For it is now certain from the phenomena of Jupiter's satellites, confirmed by the observations of different astronomers, that light is propagated in succession (NOTE: I think this means at finite speed) and requires about seven or eight minutes to travel from the sun to the earth." This is essentially the correct value.
Of course, to find the speed of light it was also necessary to know the distance from the earth to the sun. During the 1670's, attempts were made to measure the parallax of Mars, that is, how far it shifted against the background of distant stars when viewed simultaneously from two different places on earth at the same time. This (very slight) shift could be used to find the distance of Mars from earth, and hence the distance to the sun, since all relative distances in the solar system had been established by observation and geometrical analysis. According to Crowe (Modern Theories of the Universe, Dover, 1994, page 30), they concluded that the distance to the sun was between 40 and 90 million miles. Measurements presumably converged on the correct value of about 93 million miles soon after that, because it appears Römer (or perhaps Huygens, using Römer's data a short time later) used the correct value for the distance, since the speed of light was calculated to be 125,000 miles per second, about three-quarters of the correct value of 186,300 miles per second. This error is fully accounted for by taking the time light needs to cross the earth's orbit to be twenty-two minutes (as Römer did) instead of the correct value of sixteen minutes.
Eclipse is shorter
than it should be.
History_of_c-2002_Askey

7. 1728 Bradley and Stellar Aberration
History of c Askey RET summer 2002
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1728 Bradley and Stellar Aberration
The stellar aberration is approximately the ratio of the speed the earth orbits the sun to the speed of light.
Stellar aberrations cause apparent position of stars to change due to motion of Earth around sun.
Bradley used stellar aberration to calculate the speed of light by knowing:
speed of the earth around the sun.
the stellar aberration angle.
His independent confirmation, after 53 years of struggle, finally absolutely ended the opposition to a finite value for the speed of light
He calculated speed of light in a vacuum as c = km/s.
However, only Bradley's independent confirmation published January 1, 1729 ended the opposition to a finite value for the speed of light. Roemer's work, which had split the scientific community, was at last vindicated. After 53 years of struggle, science accepted the observational fact that light traveled at a finite speed. Until recently, that finite speed has been generally been considered a fixed and immutable constant of the universe in which we live.
Scientifically speaking, the velocity of light is the highest known velocity in the physical universe.Source:
FIND THE EQUATION
Starlight and Rain from website below
The next substantial improvement in measuring the speed of light took place in 1728, in England. An astronomer James Bradley, sailing on the Thames with some friends, noticed that the little pennant on top of the mast changed position each time the boat put about, even though the wind was steady. He thought of the boat as the earth in orbit, the wind as starlight coming from some distant star, and reasoned that the apparent direction the starlight was "blowing" in would depend on the way the earth was moving. Another possible analogy is to imagine the starlight as a steady downpour of rain on a windless day, and to think of yourself as walking around a circular path at a steady pace. The apparent direction of the incoming rain will not be vertically downwards-more will hit your front than your back. In fact, if the rain is falling at, say, 15 mph, and you are walking at 3 mph, to you as observer the rain will be coming down at a slant so that it has a vertical speed of 15 mph, and a horizontal speed towards you of 3 mph. Whether it is slanting down from the north or east or whatever at any given time depends on where you are on the circular path at that moment. Bradley reasoned that the apparent direction of incoming starlight must vary in just this way, but the angular change would be a lot less dramatic. The earth's speed in orbit is about 18 miles per second, he knew from Römer's work that light went at about 10,000 times that speed. That meant that the angular variation in apparent incoming direction of starlight was about the magnitude of the small angle in a right-angled triangle with one side 10,000 times longer than the other, about one two-hundredth of a degree. Notice this would have been just at the limits of Tycho's measurements, but the advent of the telescope, and general improvements in engineering, meant this small angle was quite accurately measurable by Bradley's time, and he found the velocity of light to be 185,000 miles per second, with an accuracy of about one percent. From
One of the questions raised regarding the properties of the aether
was — when material bodies move, does the aether remain stationary or is
it carried along with the bodies? In 1726 James Bradley (1693–1762)
observed that the aberration of star light could not be explained as due to
parallax and seemed to demand an aether which was not carried along
with the earth’s motion through space [Brad28]. He found that the altitude
of a star above the horizon varied with the position of the earth in its orbit
around the sun. The greatest apparent stellar displacement occurred when
the earth was moving directly toward or away from the star
I have the PERFECT WEBSITE for the diagram and content:
From Picture:Stellar aberration was first observed by the British astronomer James Bradley in Bradley was looking for parallax effects. Parallax refers to the apparent motion of nearby objects against the background of distant objects as the observer moves. (Hold a pen at arms length and look at the projection of the pen against a far wall as you move your head. The changing projection is parallax.) Knowing the amount of parallax, by measuring the angular changes with respect to the distant background, and knowing the diameter of Earth's orbit, one has sufficient information to enable a simple calculation of the distance to the nearest stars.
The parallax effect can be understood from fig   
Figure 10.3: The changing elevation, , of a star above Earth's orbital plane as Earth moves in its orbit. (Not to scale.)
The maximum elevation above Earth's orbital plane should occur at position 2 and the minimum elevation at position 4, with positions 1 and 3 yielding different lateral angles.
However, Bradley found the maximum elevation occurred at position 3 and the minimum occured at position 1. The observed effect was of the order of 40'' of arc. It turns out that the nearest stars are so far away that their parallax is less than 1''. Although Bradley didn't know the size of parallax effects, the discrepancy in the positions at which maximum and minimum elevation occurred indicated that a new phenomenon had been discovered. We call it stellar aberration. It is also observable for all stars, not just a few close ones.
The elevation, , of the star above the plane of Earth's orbit is known as the ecliptic latitude. The ecliptic is the name given to the plane of Earth's orbit and it's projection on the background stars. This is very well determined. Thus a star may be observed to describe a small ellipse in the sky, over the course of a year, if its ecliptic coordinates can be accurately measured
History_of_c-2002_Askey

8. Fizeau’s 1849 Cogwheel Experiment
History of c Askey RET summer 2002
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Fizeau’s 1849 Cogwheel Experiment
Highlights of Fizeau’s experiment:
used a slit to produce a narrow beam of light
light travels through the spaces of a cogwheel
reflecst off of a mirror
he adjusted the rotational speed of the
cogwheel until the light passes through the next
space on the wheel.
c can be calculated using the following:
c = (2D * v)/d
D = distance between the wheel and the mirror
v = the velocity of the wheel
d = the distance between spaces on the wheel
Using this method , Fizeau determined that c = 3.15 x 108 m/s.
Armand H.L. Fizeau ( ) was a French Physicist from a wealthy family.
First man to measure speed of light using purely terrestrial techniques.
Similar measurements using this set up by subsequent investigators have yielded more accurate values of c, approximately x 108 m/s.
A problem for the students: Assume the toothed wheel of the Fizeau experiment has 360 teeth and is rotating with a speed of 27.5 rev/s when the light from the source is extinquished, that is, when a burst of light passing through an opening is blocked by the adjacent tooth on return. If the distance to the mirror is 7500 m, find the speed of light. Answer: By definition of angular velocity: t = (1/720) rev / 27.5 rev/s = 5.05 x 10-5 s
These notes from Serway & Faughn (4th ed), Fundamentals of Optics
Website:
History_of_c-2002_Askey

9. Fizeau’s 1851 Water Experiment
History of c Askey RET summer 2002
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Fizeau’s 1851 Water Experiment
Mirrors send a beam of light along two different paths through moving water.
One of the paths is in the same direction as the vw, other path was opposed to the vw.
When the two paths are looked at together they produce interference patterns. Speed of light through medium is determined from these patterns.
Velocity of light in a medium is c/n, where n is the index of refraction.
Proved Fresnel's prediction that if the medium was moving an observer would measure the speed of light to be: v(light) = (c/n) + vmed(1-1/n2)
If n=1, as in a vacuum, the velocity remains unchanged.
Leads to the invariance of the c in different reference frames, a very important fact in relativity.
Armand Hippolyte Louis Fizeau ( )
His water experiment indicated that the ether is dragged along with the water though it does not have the same velocity
Ether was described as a transparent rigid, all pervasive and changeless substance (Guth39)
Fizeau used mirrors to send a beam of light along two different paths through moving water. One of the paths was in the same direction as the velocity of the water, while the other path was opposed to the water's velocity. When the two paths are looked at together they produce interference patterns. It is from these interference patterns that the speed of light through the medium is determined. The velocity of light in a medium is c/n, where n is the index of refraction. Fizeau's experiment proved Fresnel's prediction that if the medium was moving an observer would measure the speed of light to be:
v(light) = (c/n) + vm(1-1/n2)
vm is the velocity of the medium. If the refraction is one, as it is in a vacuum, the velocity remains unchanged no matter what velocity is moving. This leads to the invariance of the speed of light in different reference frames, a very important fact in relativity.
History_of_c-2002_Askey

10. Maxwell’s 1865 Theoretical Conclusion
History of c Askey RET summer 2002
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Maxwell’s 1865 Theoretical Conclusion
These equations have been tested for well over a century now, and as far as we know, they are correct and complete. Their most spectacular prediction is that changing electric and magnetic fields can produce each other by propagating as waves through space.
Maxwell's equations predict that these waves should travel at a speed which just happens to be the speed of light. He used the following equation to quantify the speed of light:
Maxwell's theory held that light is an electromagnetic oscillation, as are radio waves, microwaves, infrared waves, X-rays, and gamma rays.
Faraday predicted that light was EM waves. Look it up.
Great site:
The Electromagnetic Nature of Light
. . . we have strong reason to conclude that light itself -- including radiant heat, and other radiations if any -- is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws.
Maxwell, "Dynamical Theory of the Electromagnetic Field," 1864
Electromagnetic Waves
The most wonderful product of Maxwell's interpretation of Faraday was his conclusion that electrical and electromagnetic phenomena acted through waves. Like Faraday, Maxwell rejected the common notion that these phenomena were the product of forces that acted instantaneously between bodies at a distance, with no intervening medium. Instead, Maxwell said that there was a medium -- an "ether" -- that surrounded everything, filling all space, even a vacuum. Electricity acted through disturbances in this medium, disturbances that traveled in the form of waves. Using his equations, he showed that all such waves must be both electrical and magnetic in nature and that the speed with which the wave travels is equal to the ratio between the electrostatic and the electromagnetic units in which a given quantity of electricity is measured. This ratio was already known from experiment to yield a speed of about 300,000,000 meters per second.
This value was so close to the accepted figure for the velocity of light that Maxwell wrote in a long letter to Faraday (1861):
I think we now have strong reasons to believe, whether my theory is a fact or not, that the luminiferous and the electromagnetic medium are one
The Speed of Light
The first serious efforts to determine the speed of light were made in the 17th century, and beginning in the middle of the 19th century very careful measurements were made by a number of experimenters. After Maxwell pointed out the correlation between the figures for the speed of light and those for the electromagnetic/electrostatic ratio, many measurements were made of both in an effort to confirm experimentally their agreement.
One of Maxwell's most important achievements was his extension and mathematical formulation of Michael Faraday's theories of electricity and magnetic lines of force. His paper On Faraday's lines of force was read to the Cambridge Philosophical Society in two parts, 1855 and Maxwell showed that a few relatively simple mathematical equations could express the behaviour of electric and magnetic fields and their interrelation.
This shows how Maxwell came up with c by a theoretical approach. Connects the theoretical to the experimental physics
Great Ideas in Physics pg 129
Maxwell's Equations
(VERY GOOD WEB SITE)
These equations have been tested for well over a century now, and as far as we know, they are correct and complete. Their most spectacular prediction is that changing electric and magnetic fields can make each other by propagating as waves through space. Maxwell's equations predict that these waves should travel at the speed which just happens to be the speed of light. Light, in Maxwell's theory, is an electromagnetic oscillation, as are radio waves, microwaves, infrared waves, X-rays, and gamma rays. --source
History_of_c-2002_Askey

11. Foucault’s Method Introduced in 1875
History of c Askey RET summer 2002
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Foucault’s Method Introduced in 1875
Leon Foucault bounced light from a rotating mirror on to a stationary curved mirror. This light is then reflected off this mirror back to the rotating mirror.
Light is then deflected by a partially silvered mirror to a point where it can easily be observed. As the mirror is rotated, the light beam will focus at some displacement from s in the figure. By measuring this displacement, c can be determined from Foucault’s equation: c = (4AD2)/((A + B)s)
D is the distance from the rotating mirror to the fixed mirror,
A is the distance from L2 and L1, minus the focal length
B is the L2 and the rotating mirror
 is the rotational velocity of the mirror.
I am looking for a better drawing.
Leon Foucault is more remembered for his 1851 pendulum experiment which proved the rotation of the Earth. There is one hanging in the Omniplex
Foucault continually increased the accuracy of this method over the next 50 years. His final measurement in 1926 determined that light travelled at 299,796 Km/s.
Focault was Fizeau’s contemporary and the rotating mirror improved on Fizeau’s method
From next slide
A.A. Michelson used the Foucault method in which a beam of light is focused by a lens and then is reflected off of a rotating mirror to a fixed, curved mirror. The beam of light is then reflected off this mirror back to the rotating mirror. The light is then deflected by a partially silvered mirror to a point where it can easily be observed. As the rotating mirror revolves, the point on the fixed, curved mirror changes. This is the reason the mirror needs to be curved. Even as the point of light moves it will always be reflected back to the rotating mirror so that it can be observed. As the mirror is rotated, the light beam will focus at some displacement from s in the figure. By measuring this displacement, c can be determined from Foucault’s equation:
c = (4AD2)/((A + B)s)
D is the distance from the rotating mirror to the fixed mirror,
A is the distance from L2 and L1, minus the focal length
B is the L2 and the rotating mirror
 is the rotational velocity of the mirror.
Using this method, Michelson was able to calculate c = 299,792 km/s.
History_of_c-2002_Askey

12. Michelson’s 1878 Rotating Mirror Experiment
History of c Askey RET summer 2002
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Michelson’s 1878 Rotating Mirror Experiment
German American physicist A.A. Michelson realized, on putting together Foucault’s apparatus, that he could redesign it for much greater accuracy.
Instead of Foucault's 60 feet to the far mirror, Michelson used 2,000 feet..
Using this method, Michelson was able to calculate c = 299,792 km/s
. 20 times more accurate than Foucault
. Accepted as the most accurate measurement of c for the next 40 years. I
Albert A. Michelson, German American Physicist ( )
Michelson won the Nobel Prize in 1907 for his efforts
1907 Nobel Laureate in Physics
“for his optical precision instruments and the spectroscopic and metrological investigations carried out with their aid.”
Background
Place of Birth: Strelno, then Germany
Residence: U.S.A
Affiliation: Chicago University
Albert Michelson was born in 1852 in Strzelno, Poland. His father Samuel was a Jewish merchant, not a very safe thing to be at the time. Purges of Jews were frequent in the neighboring towns and villages. They decided to leave town. Albert's fourth birthday was celebrated in Murphy's Camp, Calaveras County, about fifty miles south east of Sacramento, a place where five million dollars worth of gold dust was taken from one four acre lot. Samuel prospered selling supplies to the miners. When the gold ran out, the Michelsons moved to Virginia City, Nevada, on the Comstock lode, a silver mining town. Albert went to high school in San Francisco. In 1869, his father spotted an announcement in the local paper that Congressman Fitch would be appointing a candidate to the Naval Academy in Annapolis, and inviting applications. Albert applied but did not get the appointment, which went instead to the son of a civil war veteran. However, Albert knew that President Grant would also be appointing ten candidates himself, so he went east on the just opened continental railroad to try his luck. Unknown to Michelson,Congressman Fitch wrote directly to Grant on his behalf, saying this would really help get the Nevada Jews into the Republican party. This argument proved persuasive. In fact, by the time Michelson met with Grant, all ten scholarships had been awarded, but the President somehow came up with another one. Of the incoming class of ninety-two, four years later twenty-nine graduated. Michelson placed first in optics, but twenty-fifth in seamanship. The Superintendent of the Academy, Rear Admiral Worden, who had commanded the Monitor in its victory over the Merrimac, told Michelson: "If in the future you'd give less attention to those scientific things and more to your naval gunnery, there might come a time when you would know enough to be of some service to your country."
On returning to Annapolis from the cruise, Michelson was commissioned Ensign, and in 1875 became an instructor in physics and chemistry at the Naval Academy, under Lieutenant Commander William Sampson. Michelson met Mrs. Sampson's niece, Margaret Heminway, daughter of a very successful Wall Street tycoon, who had built himself a granite castle in New Rochelle, NY. Michelson married Margaret in an Episcopal service in New Rochelle in 1877.
At work, lecture demonstrations had just been introduced at Annapolis. Sampson suggested that it would be a good demonstration to measure the speed of light by Foucault's method. Michelson soon realized, on putting together the apparatus, that he could redesign it for much greater accuracy, but that would need money well beyond that available in the teaching demonstration budget. He went and talked with his father in law, who agreed to put up $2,000. Instead of Foucault's 60 feet to the far mirror, Michelson had about 2,000 feet along the bank of the Severn, a distance he measured to one tenth of an inch. He invested in very high quality lenses and mirrors to focus and reflect the beam. His final result was 186,355 miles per second, with possible error of 30 miles per second or so. This was twenty times more accurate than Foucault, made the New York Times, and Michelson was famous while still in his twenties. In fact, this was accepted as the most accurate measurement of the speed of light for the next forty years, at which point Michelson measured it again. from
Picture credit
History_of_c-2002_Askey

13. The Michelson Interferometer
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The Michelson Interferometer
Monochromatic light split and sent it along 2 different paths to the same detector where the 2 waves will constructively or destructively interfere
If one path is an integral number of half-wavelengths longer than the other, then the waves will interfere constructively and will be bright at the detector.
Otherwise, there will be alternating patches of light and dark areas called interference fringes.
The wavelength of the radiation in the interferometer can be determined from:  = 2 L/N L is the length increase of one path,
N is the number of maxima observed during the increase.
History_of_c-2002_Askey

14. 1887 Michelson-Morley Experiment
History of c Askey RET summer 2002
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1887 Michelson-Morley Experiment
Michelson and Morley experiment produced a null result in regards to ether wind
Theoretical implications of this
result is that the equations for the electromagnetic field must by their very nature reflect the indifference to the ether’s motion.
This implies that Maxwell’s equations must remain invariant under the transformation from one reference system to another.
The very best web site on the subject:
From Jack Meadows, The Great Scientists
History_of_c-2002_Askey

15. 1891: Blondlot’s Parallel Wires
History of c Askey RET summer 2002
1891: Blondlot’s Parallel Wires
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Selected frequencies were transmitted along a pair of parallel wires and reflected at the far end.
This created a system of stationary waves with nodes and antinodes spaced a regular intervals.
Still looking for good information on this one.
He determined c = 297,600 km/s +/
Rene Blondlot’s
From
Trapping Radio Waves
" Electromagnetic radiation predicted by Maxwell was produced experimentally by Hertz and he conducted experiments which showed that they traveled with a finite velocity. A value was first obtained by Blondlot. "
Selected frequencies were transmitted along a pair of parallel wires and reflected at the far end. This created a system of stationary waves with nodes and antinodes spaced a regular intervals. Knowing the frequencies and the distances between nodes, the speed of the radiation could be determined.
" Thirteen different frequencies between 10x106 and 30x106 Hz were employed, the total spread of the values of velocity was 5% and the average value was 297,600 km/sec. Blondlot concluded that the value agreed with the velocity of light and with the ratio of e.s.u and e.m.u. units within the accuracy of the experiment. "
(source: Froome and Essen, "The Velocity of Light and Radio Waves", Academic Press, 1969)
Today Blondlot is mostly remembered for his unfortunate "discovery", in 1903, of N-Rays. N-Rays enjoyed a short popularity but were ultimately shown not to exist.
Knowing the frequencies and the distances between nodes, the speed of the radiation could be determined.
Blondlot’s determined c = 297,600 km/sec.
History_of_c-2002_Askey

16. L. Essen’s 1950 Microwave Cavity Resonator
History of c Askey RET summer 2002
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L. Essen’s 1950 Microwave Cavity Resonator
Essen used radiation to produce standing waves in a closed hollow metal cylinder
He produced radiation with resonant frequencies of 9.5 GHz, 9 GHz, and 6 GHz
wavelength of the radiation in free space is determined by:(1/)2 = (/D)2 + (n/2L)2
D is the diameter of the cylinder
L is the length
n is the # of half-wavelengths inside the cavity
 is obtained from solving wave equations
Essen used this method to determine c
c = 299,792.5  3 km/s using c = 
 is the resonant frequency
 is the wavelength in free space.
Add the wave equation
If radiation is sent through a closed hollow metal cylinder and the length of the cylinder is an integral number of half-wavelengths, resonance is obtained and standing waves are produced. The wavelength in free space is different from the wavelength in the cavity, but at resonance, the wavelength of the radiation in free space is determined by:(1/)2 = (/D)2 + (n/2L)2 where D is the diameter of the cylinder, L is the length, n is the number of half-wavelengths inside the cavity, and  is obtained from solving the wave equations for electric and magnetic fields in a cylindrical cavity. Essen used this method to determine c. He produced radiation with resonant frequencies of 9.5 GHz, 9 GHz, and 6 GHz, measured the dimensions of the cavity at resonance, and determined the corresponding wavelengths.
c = 299,792.5  3 km/s using c = 
 is the resonant frequency
 is the wavelength in free space.
History_of_c-2002_Askey

17. Froome’s 1958 Four-Horn Microwave Interferometer
History of c Askey RET summer 2002
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Froome’s 1958 Four-Horn Microwave Interferometer
Froome generated 72 GHz radiation and sent it through his interferometer.
Radiation divided into two beams, sent through two identical waveguides and out to two receivers on a movable cart.
Moving the receiver changed the path lengths of the two beams and caused interference in the detector.
Every half-wave displacement in receiver, showed constructive interference.
He determined the free space wavelength () of the radiation by:
N /2 = z + A(1/z1 - 1/z2)
N is the number of interference minima
A is a constant
z = z1 - z2 is the displacement of the cart.
He calculated c = 299,792.5  0.3 km/s.
This speed of light measurement was limited primarily in the difficulty in measuring the very long wavelength (about 0.4 cm) of the 72 GHz radiation. Clearly a better measurement would result if higher frequencies could be employed, where wavelengths could be more accurately measured. From Matt’s Article
History_of_c-2002_Askey

18. 1983 Breakthrough by Boulder Group: Meter Redefined
History of c Askey RET summer 2002
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1983 Breakthrough by Boulder Group: Meter Redefined
Signals synthesized at progressively higher and higher frequencies using harmonics generation and mixing methods to lock the frequency of a nearby oscillator or laser to the frequency of this synthesized signal.
Photodiodes and metal-insulator-metal diodes used for harmonic generation
A frequency chain was constructed linking a microwave output of the cesium frequency so the group could directly measure the frequency of a helium-neon laser stabilized against the 3.39 µm transition of methane.
Resulted in a reduction in the uncertainty of speed of light by a factor of 100
Formed basis for a new definition of the meter based on the speed of light.
“The meter is the length of the path traveled
by light in a vacuum during the time interval
of 1/ of a second.”.
Led to the development of high resolution spectroscopic methods.
From Matts Article he gave you 7/22
At this point, I will just scan the picture of the Boulder Group. -- Matt says no to the pic
New definition of the meter was accepted by the 17th Conference Generale des Poids et Mesures, in 1983 as simply “The meter is the length of the path traveled by light in a vacuum during the time interval of 1/ of a second.”.
Now the speed of light is constant. It will never have to be measured again.
Matt’s article talks about all the benefits of this greater accuracy from the Boulder group.
Best Web site:
History_of_c-2002_Askey

19. Historical Accuracy of speed of light
History of c Askey RET summer 2002
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Historical Accuracy of speed of light
I did this on Excel. My first time to use Excel so I’m sure it could be done much less primitive.
History_of_c-2002_Askey

20. Classroom Application: Microwaving Marshmallows
History of c Askey RET summer 2002
7/24
Classroom Application: Microwaving Marshmallows
Without rotating trays and reflecting fan, microwave ovens cook unevenly.
A pattern of standing waves forms inside the oven chamber.
Creates an array of hotspots throughout the oven's volume.
An operating frequency of 2450 MHz produces a wavelength of 12.2cm.
Hotspots should be at halfwave points, or approximately every 6 cm, but in a complex 3D pattern.
After about one minute on low power, a one layer sheet of small marshmallows should have melt spots that resemble the pattern behind this text.
These notes are from sites I have found on the internet.
Marshmallows-A Take-Home Lab
from Robert H. Stauffer, Jr., Cimarron-Memorial High School, Las Vegas, Nevada, USA
The activity requires a microwave oven, a microwave-safe casserole dish, a bag of marshmallows, and a ruler. (The oven must be of the type that has no mechanical motion-no turntable or rotating mirror. If there is a turn-table, remove it first.) First, open the marshmallows and place them in the casserole dish, completely covering it with a layer one marshmallow thick. Next, put the dish of marshmallows in the microwave and cook on low heat. Microwaves do not cook evenly and the marshmallows will begin to melt at the hottest spots in the microwave. (I leaned this from our Food Science teacher Anita Cornwall.) Heat the marshmallows until they begin to melt in four or five different spots. Remove the dish from the microwave and observe the melted spots. Take the ruler and measure the distance between the melted spots. You will find that one distance repeats over and over. This distance will correspond to half the wavelength of the microwave, about 6 cm. Now turn the oven around and look for a small sign that gives you the frequency of the microwave. Most commercial microwaves operate at 2450 MHz.
All you do now is multiply the frequency by the wavelength. The product is the speed of light.
Example:
Velocity = Frequency ´ Wavelength
Velocity = 2450 MHz ´ m
Velocity = 2.99 ´ 108 m/s
This works in my physics class, often with less than 5% error. Then the students can eat the marshmallows.
Procedure
1. First open the marshmallows and place them on the casserole dish, completely covering it with a layer one marshmallow thick.
2. Next put the dish of marshmallows in the microwave and cook on low heat. The microwave does not cook evenly and the marshmallows will begin to melt at the hottest spots in the microwave.
3. Heat the marshmallows until they begin to melt in 4 or 5 different spots.
4. Remove the dish from the microwave oven and observe the spots.
5. Take the ruler and measure the distance between the melted spots. You will soon find that one distance repeats over and over. This distance will correspond to the wavelength of the microwave. That should be a little over 12 centimeters.
6. Now, turn the microwave around and look for a small sign that tells you the frequency of the microwave. Most commercial microwaves operate at 2450 MHz.
NOTE: Only do one trial. Obtain the wavelength data from other lab groups. If no frequency is visible on the microwave, use 2450 MHz.
Why does a Microwave Oven have Hot Spots?
So when we did the experiment with marshmallows in a microwave oven I could see that the "hot" and "cold" spots made a regular pattern, but I don't understand why it's kind of like a checkerboard. If it was just the crests and troughs of the waves causing the hot spots, I would have thought the pattern would look like this:
Well, that is what it would look like if the waves were only coming from one direction, but that's not how microwaves work. If the wave you imagined was moving from side to side in the microwave, there is also a wave going front to back, like this:
Oh, and now we have to combine the two waves, like we had to in the Two Slit Experiment, except here we have to add two whole surfaces, not just two lines...
Exactly. It's a little harder to visualize, but its the same kind of thing. The result looks like this:
Cool! The high and low areas in that picture match the pattern we saw in the marshmallow experiment. But wait a second, I have two problems with this. First, if the waves are moving at the speed of light, shouldn't the hot spots be moving, too? Also, does this mean that all microwave ovens have TWO "microwave guns"? I mean, one for the back and forth waves and one for the front to back waves?
Not only are those both good questions, but they have the same answer. The "microwave guns" are called "magnetrons" and a microwave oven only has one of them. We get two different wave patterns because the physics of a microwave oven leads to something called Standing Waves...
History_of_c-2002_Askey

21. History of c Askey RET summer 2002
Conclusion
7/24
Why would so many scientists throughout the last four centuries spend so much of their careers to make an accurate measurement of the speed of light?
A small error in c causes an enormous error in distance measurements to stars.
Einstein's theory of relativity would not be possible without first discovering that c is invariant in different reference frames.
These experiments eventually led to the redefinition of the meter in 1983
I will add a couple of more benefits and then add the watermark to the back.
History_of_c-2002_Askey

22. Bibliography
Fishbane, P., S. Gasiorowitz, and S. Thornton. Physics for Scientists and Engineers. New Jersey: Prentice Hall, 1993.
Froome, K., and L. Essen. The Velocity of Light and Radio Waves. London: Academic Press, 1969.
Halliday, D., R. Resnick, and J. Walker. Fundamentals of Physics. New York: John Wiley & Sons, 1993.
Michelson, A. Experimental Determination of the Velocity of Light. Minneapolis: Lund Press, 1964.
Mulligan, J. Introductory College Physics. New York: McGraw-Hill Book Co., 1985.
Resnick, R., and D. Halliday. Basic Concepts of Relativity. New York: MacMillan Publishing Company, 1992.
Serway, R.A., and Faughn J.S.. College Physics. Florida: Harcourt,Brace& Co., 1999
Sobel, D. and Andrewes, W.J., The Illustrated Longitude. New York: Walker Publishing, 1998
Sullivan, D.B., Speed of Light From Direct frequency and Wavelength Measurements. Matt’s Article he gave me on 7/22