Speed of Light
The gap we experience between seeing lightning and hearing thunder shows that
sound travels relatively slowly. Light seems to travel so fast that to our experience its
speed seems infinite; that is, we seem to observe events at the instant they happen.
There are many scientists who were interested to determine the speed of light. You
are required to play the role of a historian to document how the speed of light was
determined by the following 3 scientists, describing their successes, failures,
accuracies of their experiments and uniqueness of each of their findings.
1) Galileo Galilei
successes, failures, accuracies of their experiments and uniqueness
Although Galileo was the first person of record to try to determine the speed of light, he was not
successful. His experiments took place over terrestrial distances and the timing methods available to
him were far to crude to make a successful determination given such distances and the very great
speed of light.
Galileo attempted to measure the speed of light, For measurements of particularly short intervals of
time, Galileo sang songs with whose timing he was familiar. However he concluded that his
measurement technique was too imprecise to accurately determine its value. He climbed one hill and
had an assistant to climb another hill; both had lanterns with shutters, initially closed. He then opened
the shutter of his lantern. His assistant was instructed to open his own shutter upon seeing Galileo's
lantern. Galileo then measured the time interval for his assistant's shutter to open. Knowing the time
interval and the separation between the hills, he determined the apparent speed of light.
On repeating the experiment with more distant hills, Galileo obtained the same time lapse, concluding
that the time for the light to travel was much less than his and his assistant's reaction time, and
therefore that the actual speed of light was beyond the sensitivity of his measurement techniques.
Galileo showed a remarkably modern appreciation for the proper relationship between mathematics,
theoretical physics, and experimental physics.
For example: He understood the parabola, both in terms of conic sections and in terms of the ordinate
(y) varying as the square of the abscissa (x). He asserted that the parabola was the theoretically-ideal
trajectory for uniformly accelerated motion, in the absence of friction and other disturbances. Further,
he noted that there are limits to the validity of this theory, stating that it was appropriate only for
laboratory-scale and battlefield-scale trajectories, and noting on theoretical grounds that the parabola
could not possibly apply to a trajectory so large as to be comparable to the size of the planet. He
recognized that his experimental data would never agree exactly with any theoretical or mathematical
form, because of the imprecision of measurement, irreducible friction, and other factors.
2) Olaus Roemer
successes, failures, accuracies of their experiments and uniqueness
Olaus Roemer, who, in 1676, first successfully measured the speed of light. His method was based
on observations of the eclipses of the moons of Jupiter .
Roemer noted that the observed time interval between successive eclipses of a given moon was
about seven minutes greater when the observations were carried out when the earth in its orbit was
moving away from Jupiter than when it was moving toward Jupiter. He reasoned that, when the earth
was moving away from Jupiter, the observed time between eclipses was increased above the true
value (by about 3.5 minutes) due to the extra distance that the light from each successive eclipse had
to travel to reach the earth. Conversely, when the earth was moving toward Jupiter, the observed
interval between eclipses was decreased (by about 3.5 minutes) because of the decreased distance
that the light had to travel on each successive eclipse.
Had the earth not been moving, the light from successive eclipses would have to travel the same
distance to the earth, so that the true interval between eclipses would be observed. However, when
the earth was moving away from Jupiter, the light had to travel a greater distance to reach the earth
from each successive eclipse, and conversely a smaller distance when the earth was moving toward
Jupiter. Since the speed of the earth in its orbit was known, the distance that the earth had moved
between eclipses could be calculated. The speed of light was then estimated to account for the seven
minute overall variation of the observed interval between successive eclipses.
Roemer's estimate for the speed of light was 140,000 miles/second, which is remarkably good
considering the method employed.
3) Albert A. Michelson
successes, failures, accuracies of their experiments and uniqueness
Michelson looked at the past apparatus to measure light. On putting together the apparatus, he felt
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.