How do scienmtists measure the speed of light?

Hello.

Scientists have used multiple methods to measure the speed of light.
There's an excellent overview of the history of the various methods on
this web page:
"Speed of Light"
http://www.mic-d.com/curriculum/lightandcolor/lightspeed.html

As indicated on that web page, some of the earliest calculations were
based on astronomy.  Modern scientists have used lasers to get very
precise measurements.

"By the late 1960s, lasers were becoming stable research tools with
highly defined frequencies and wavelengths. It quickly became obvious
that a simultaneous measurement of frequency and wavelength would
yield a very accurate value for the speed of light, similar to an
experimental approach carried out by Keith Davy Froome using
microwaves in 1958. Several research groups in the United States and
in other countries measured the frequency of the 633-nanometer line
from an iodine-stabilized helium-neon laser and obtained highly
accurate results. In 1972, the National Institute of Standards and
Technology employed the laser technology to measure the speed at
299,792,458 meters per second (186,282 miles per second), which
ultimately resulted in the redefinition of the meter with a highly
accurate estimate for the speed of light."
http://www.mic-d.com/curriculum/lightandcolor/lightspeed.html


Earlier methods, though not as accurate, were quite fascinating.  Two
pioneers in this field were French scientists Armand Fizeau and
Jean-Bernard-Leon Foucault.

"In 1849, Fizeau engineered a device that flashed a light beam through
a toothed wheel... and then onto a fixed mirror positioned at a
distance of 5.5 miles away. By rotating the wheel at a rapid rate, he
was able to steer the beam through a gap between two of the teeth on
the outward journey and catch reflected rays in the neighboring gap on
the way back. Armed with the wheel speed and distance traveled by the
pulsed light, Fizeau was able to calculate the speed of light."
http://www.mic-d.com/curriculum/lightandcolor/lightspeed.html


"Foucault employed a rapidly rotating mirror driven by a compressed
air turbine to measure the speed of light. In his apparatus... a
narrow beam of light is passed through an aperture and then through a
glass window (acting also as a beam splitter) with a finely graduated
scale before impacting on the rapidly spinning mirror. Light reflected
from the spinning mirror is directed through a battery of stationary
mirrors in a zigzag pattern designed to increase the pathlength of the
instrument to about 20 meters without a corresponding increase in
size. In the amount of time it took the light to reflect through the
series of mirrors and return to the rotating mirror, a slight shift in
the mirror position had occurred. Subsequently, light reflected from
the shifted position of the spinning mirror follows a new pathway back
to the source and into a microscope mounted on the instrument. The
tiny shift in light could be seen through the microscope and recorded.
By analysis of the data collected from his experiment, Foucault was
able to calculate the speed of light as 298,000 kilometers per second
(approximately 185,000 miles per second)."
http://www.mic-d.com/curriculum/lightandcolor/lightspeed.html

Also see:
"Speed of Light Demonstration by the Foucault Method"
http://www.pas.rochester.edu/~pavone/particle-www/summer2002/FoucaultDemonstration.htm


More information:

"How is the speed of light measured?"
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html

"The Speed of Light"
http://www.brooklyn.cuny.edu/bc/ahp/CellBio/SBAM/SBAM.Speed.html

A basic description of a laser method may be seen here:
http://store.pasco.com/pascostore/showdetl.cfm?&DID=9&Product_ID=51875&Detail=1

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I hope this helps.

Although this is alluded to in some of the links juggler-ga provided,
it is useful to repeat that in 1983 the meter was redefined in terms
of the speed of light:

"The meter is the length of the path travelled by light in vacuum
during a time interval of 1/299 792 458 of a second."
http://physics.nist.gov/cuu/Units/meter.html

Thus it is actually no longer possible to measure the speed of light,
because it is fixed by the definitions of the units.

If you repeat an experiment that used to measure the speed of light,
you're actually measuring the ratio of your distance reference and
your time reference.

This is why Ken Evenson titled his talk about the last speed-of-light
experiments "The Final Measurement of the Speed of Light."  (Pretty
cool when you think about it: after 300 years of experiments, they did
such a good job that nobody can improve upon it!)

Adding to the excellent input from others, one should appreciate that
light doesn?t really have a speed. It is observed in our terms to have
speed, but as far as it is concerned it travels from A to B instantly.
The mooted ?speed of light? is not a ?speed?, it is an attribute of
photons, as is for instance ?spin? of an electron.

By the same token, it is kinda imprecise to say that a distant event,
say a hundred light years away, happened one hundred years ago when we
finally see it. It is valid, and more accurate, to say that it happens
when the light reaches us. And if you travel at the speed of light,
the third dimension doesn?t exist, so you arrive instantly. Of course,
returning back home you will find that time has elapsed there.

Hope this makes sense.

Best