DETERMINATION OF SPEED OF LIGHT
Today, 7th December, 2016, we celebrate 340th anniversary of the determination of the speed of light. It was on December 7th, in the year 1676, that Olaus
Roemer succeeded in his experiment of determining the speed of light.
Earlier Light is considered to travel with the greatest speed in the Universe. Later it was found to be not true. We can look into some earlier attempts to determine the speed of light.
OLAUS ROEMER
CLICK HERE TO READ
How
is the speed of light measured?
Before the seventeenth century, it was
generally thought that light is transmitted instantaneously. This was
supported by the observation that there is no noticeable lag in the position of
the Earth's shadow on the Moon during a lunar eclipse, which would otherwise be
expected if c were finite. Nowadays, we know that light moves just
too quickly for the lag to be noticeable. Galileo doubted that light's
speed is infinite, and he devised an experiment to measure that speed by
manually covering and uncovering lanterns that were spaced a few miles
apart. We don't know if he ever attempted the experiment, but again c
is too high for such a method to give an even remotely accurate answer.
The first successful measurement of c was made by Olaus
Roemer in 1676. He noticed that, depending on the Earth–Sun–Jupiter
geometry, there could be a difference of up to 1000 seconds between the
predicted times of the eclipses of Jupiter's moons, and the actual times that
these eclipses were observed. He correctly surmised that this is due to the
varying length of time it takes for light to travel from Jupiter to Earth as
the distance between these two planets varies. He obtained a value of c
equivalent to 214,000 km/s, which was very approximate because planetary
distances were not accurately known at that time.
In 1728 James Bradley made another estimate by observing stellar
aberration, being the apparent displacement of stars due to the motion of the
Earth around the Sun. He observed a star in Draco and found that its
apparent position changed throughout the year. All stellar positions are
affected equally in this way. (This distinguishes stellar aberration from
parallax, which is greater for nearby stars than it is for distant
stars.) To understand aberration, a useful analogy is to imagine the
effect of your motion on the angle at which rain falls past you, as you run
through it. If you stand still in the rain when there is no wind, it
falls vertically on your head. If you run through the rain, it comes at
you at an angle, and hits you on the front. Bradley measured this angle
for starlight, and knowing the speed of the Earth around the Sun, he found a
value for the speed of light of 301,000 km/s.
The first measurement of c that didn't make use of the
heavens was by Armand Fizeau in 1849. He used a beam of light reflected
from a mirror 8 km away. The beam was aimed at the teeth of a rapidly
spinning wheel. The speed of the wheel was increased until its motion was
such that the light's two-way passage coincided with a movement of the wheel's
circumference by one tooth. This gave a value for c of 315,000
km/s. Leon Foucault improved on this result a year later using rotating
mirrors, which gave the much more accurate value of 298,000 km/s. His
technique was good enough to confirm that light travels slower in water than in
air.
After Maxwell published his theory of
electromagnetism, it became possible to calculate the speed of light indirectly
by instead measuring the magnetic permeability and electric permittivity of
free space. This was first done by Weber and Kohlrausch in 1857. In
1907 Rosa and Dorsey obtained 299,788 km/s in this way. It was the most
accurate value at that time.
Many other methods were subsequently employed to further
improve the accuracy of the measurement of c, so that it soon became
necessary to correct for the refractive index of air since c is light's
speed in a vacuum. In 1958 Froome obtained a value of 299,792.5 km/s
using a microwave interferometer and a Kerr cell shutter. After 1970 the
development of lasers with very high spectral stability and accurate caesium
clocks made even better measurements possible. Up until then, the
changing definition of the metre had always stayed ahead of the accuracy in
measurements of the speed of light. But by 1970 the point had been
reached where the speed of light was known to within an error of plus or minus
1 m/s. It became more practical to fix the value of c in the
definition of the metre and use atomic clocks and lasers to measure accurate
distances instead. Nowadays, the speed of light in vacuum is defined to
have an exact fixed value when given in standard units. Since 1983 the
metre has been defined by international agreement as the distance travelled by
light in vacuum during a time interval of 1/299,792,458 of a second. This
makes the speed of light exactly 299,792.458 km/s. (Also, because the
inch is now defined as 2.54 centimetres, the speed of light also has an exact
value in imperial units.) This definition only makes sense because the
speed of light in vacuum is measured to have the same value by all observers; a
fact which is subject to experimental verification. Experiments are still
needed to measure the speed of light in media such as air and water.
No comments:
Post a Comment