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Galileo
Galileo came up with an experiment to measure light's velocity: he and his assistant each took a shuttered lantern, and they stood on hilltops one mile apart. Galileo flashed his lantern, and the assistant was supposed to open the shutter to his own lantern as soon as he saw Galileo's light. Galileo would then time how long it took before he saw the light from the other hilltop -
Period: to
Speed of Light
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Romer
Ole Rømer first demonstrated in 1676 that light travelled at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io. -
James Bradley
The aberration of light is an astronomical phenomenon which produces an apparent motion of celestial objects about their real locations. It was discovered and later explained by the third Astronomer Royal, James Bradley, in 1729, who attributed it to the finite speed of light and the motion of Earth in its orbit around the Sun -
Armand Fizeau
Armand Fizeau made the first land-based estimate in 1849. Fizeau set up an experiment where he placed a light five and a half miles away from a mirror. In front of the light, he set up a wheel with toothed edges. When the light would shine, it would pass through the gap between two teeth. The mirror would reflect the beam back and would shine through the next gap between the teeth. Fizeau could then estimate the speed of light from the speed at which the toothed wheel was moving. He measured the -
Albert michelson
Rotating Mirror Experiment
German American physicist A.A. Michelson realized, on putting togetherFoucault’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. -
Perot/ Fabry
Perot interferometer makes it possible to transform the incident beam of light so that the major part of energy in the reflected wave is contained in the side lobes in the far-field diffraction pattern and radiation in the zero order of diffraction along the optical axis is suppressed -
Rosa/Dorsey
After Maxwell published his theory of electromagnetism it became possible to calculate the speed of light indirectly from the magnetic permeability and electric permitivity 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 -
Karolus & Mittelstaed
In 1928 Karolus & Mittelstaedt used a Kerr cell to modulate a beam of light to measure its speed. Earlier measurements had used mechanical means of modulation achieving frequencies of around 10 kHz, but the Kerr cell allow frequencies of 10 MHz and greater precision of measurement -
Francis G. Pease and Fred Pearson
1930, Michelson began a collaboration with Francis G. Pease and Fred Pearson to perform a measurement in a 1.6 km tube at Pasadena, California -
Essen/Gordon-Smith
In 1947, Louis Essen and A.C. Gordon-Smith used a microwave cavity of precisely known dimensions to establish the frequency for a variety of normal modes of microwaves. As the wavelength of the modes was known from the geometry of the cavity and from electromagnetic theory, knowledge of the associated frequencies enabled a calculation of the speed of light. -
Froome
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
– Every half-wave displacement in receiver,
showed constructive interference. -
Evenson
The modern method for determining the speed of light was developed by K. M. Evenson et al. It relies on separate mesurements of wavelength and frequency emitted by a stabilized laser. They achieved the value c=2 999 792 458 m/s, which has uncertainty of only 1 m/s
