Home • About • FAQs • News • Random Expert Selector • Random Question Selector • Contact Us • 

Top Level >

Physics

hry India
How was the speed of light calculated?

 
Discuss this question
Discuss Answer

The first successful measurement of c was made by Olaus Roemer in 1676. He noticed that the time between the eclipses of the moons of Jupiter was less as the distance away from Earth is decreasing than when it is increasing. 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 changes. He obtained a value 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 during the year. All stellar positions are affected equally in this way. This distinguishes the effect from parallax which affects nearby stars more noticeably. A useful analogy to help understand aberration is to imagine the effect of motion on the angle at which rain falls. If you stand still in the rain when there is no wind it comes down vertically on your head. If you run through the rain it appears to come at you from an angle and hit you on the front. Bradley measured this angle for starlight. 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 on Earth was by Armand Fizeau in 1849. He used a beam of light reflected from a mirror 8 km away. The beam passed through the gaps between teeth of a rapidly rotating wheel. The speed of the wheel was increased until the returning light passed through the next gap and could be seen. Then c was calculated to be 315,000 km/s. Leon Foucault improved on this a year later by using rotating mirrors and got the much more accurate answer 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 from 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 employed to improve accuracy further. It soon became necessary to correct for the refractive index of air. In 1958 Froome had the 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 kept ahead of the accuracy in measurements of the speed of light. Then the point was 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.

The actual speed of light is 299,792.458 km/s.

Prof Benedict Weber
(Expert Rating 2424)
   Member has an expert rating of 2000+
Discuss Answer

According to the currently prevailing definition, adopted in 1983, the speed of light is exactly 299,792,458 metres per second (approximately 3 × 108 metres per second, or about thirty centimetres (one foot) per nanosecond). The value of c defines the permittivity of free space (ε0) in SI units as:

http://en.wikipedia.org/wiki/Speed_of_light

mistakenidentity
(Expert Rating 2444)
   Member has an expert rating of 2000+
Discuss Answer

The most recent method is to measure the length of time it takes for a beam of light to travel to the mirror and back to earth again. We're able to do this because astronauts placed a mirror there for that purpose. The measured result is that it takes the beam of light slightly less that 2 and a half seconds which means that it travels at 300 kilometres per second.

Journo
(Expert Rating 5820)
   Member has an expert rating of 5000+
Discuss Answer

The 1st good experiment followed the basic ideas of Galileo which was to note how long it took for light to travel a known distance. Galileo's problem was he did not have a great enough distance to look at. Then some time in the 1600s not long after Galileo, Roemer noticed that Jupiter's moon Io seemed to be in the "wrong" place even though it's orbit time around Jupiter was the same. The difference was that the earth was closer or farther depending on when the measurement was made. Using this so-called ahead or behind time value and knowing that the orbit was a constant, he was able to calculate the difference as the speed of light coming up with 300,000 km/sec. In the 20th century a more accurate number was calculated using a laser shot from earth to a mirror placed on the moon. This measured the time it took for the light to make the known round trip. The value still is around 300,000 km/sec just more accurately known. In fact all electromagnetic radiation travels at the speed of light which is now considered to be the speed limit in the universe.

Dr Kenneth Scott USA
(Expert Rating 667)
   Member has an expert rating of 500+
Discuss Answer

My apologies that, in my previous answer to this question, I left out the rather important fact that the mirror was placed on THE MOON by astronauts and that it was by measuring the time taken for a beam of light to travel to the moon and back that gives the speed of light.

I can only plead a momentary tiredness, brought about by excessive amounts of mince pie and Christmas sherry!

Journo
(Expert Rating 5820)
   Member has an expert rating of 5000+
 Top of Page Terms and Conditions Privacy Disclaimer

© Simply Explained