Speed of Light
Armand H. Louis Fizeau
Armand H. Louis Fizeau was a famous French physicist. He was born in the city of Paris (France) on September 23, 1819. Louis’s earliest work was basically focused on improvements of photographic processes. In year 1849 by measuring speed of light he became the first one to have such measurements taken on earth. In this process he had used a narrow ray of light which was reflected by a mirror placed approximately eight kilometer away. He died on 18 September, 1896 at Venteuil .
Objective
The primary objective of this experiment was to measure the speed of light on land with better confidence interval than previous observations .
Apparatus
The Fizeau experiment's apparatus consisted of light reflecting from rotating mirror to another stationary mirror. Fizeau took as base line the displacement between two small hills around Paris named as Montmartre and Suresnes which was measured around 8,633 meters. He also arranged for the observation of returning rays of light as the result, he conducted his experiment with the total displacement of about 17,266 meters .
Fizeau Apparatus
Method
As the first mirror moved slightly while in the meantime the light bounced back to the stationary mirror and returned back to first mirror it was deflected across from original source by making small angle. Fizeau also used small rotating wheel having 720 teeth it was driven with the help of clockwork that Froment had made. Light from a source was directed on the wheel focusing on its rim then it was converted into parallel light beam with the help of a telescope which was traversed to 8,633 meters. At that point it was received through another telescope and it focused the light beam to another concave mirror while sending light back to the same path from which it started initially. Now the light back to the initial point was observed between the rim and teeth of the wheel. This system was entirely focused on the wheel to be at rest while the light is assumed to be shining in between the small gaps which were placed in between the teeth of the wheel. After that wheel was rotated automatically converting the light beam into series of light flashes .
Data Collected
As the rotation rate of the wheel was decreased the light observed to intensity dropped it occurs until the flash that has crossed in between the teeth to its way back struck to the next tooth at its return. The intensity of observed light was measure as its minimum. However by increasing rotation rate of the wheel a time came when the returning back flash light went by the next teeth gap to one it passed by on way out. This condition resulted as maximum intensity. Fizeau found that first minimum achieve by the rotating wheel was 12.6 rotation per second while first maximum at 25.2 rotation per second .
Calculations
The flash had traveled 17,266 Km in 1/ (720 x 25.2) seconds. This means that only in one second on clock light traveled a distance of 17,266 x 25.2 x 720 Kilometers so it gives Fizeau’s the exact measure of speed of light as 313,300 kilo meter per second .
Conclusion
Fizeau experiment helps in determination of the speed of light the purpose of experiment was to measure speed of light in the lab by opposing the determination of light speed by using of astronomical data for interpretation. This experiment gave reasonably closer value of speed of light at that time.
References
Hariharan, P. (2003). Optical Interferometry. Waltham: Academic Press.
Hephaestus Books. (2011). Articles on Optical Metrology, Including: Fizeau "Foucault Apparatus, Streak Camera, Ellipsometry, Optical Autocorrelation, Fourier Profilometry, Densitometry, Frequency-Resolved Optical Gating. Panem: Hephaestus Books.
Hockey, T., Bolt, M., Palmeri, J., & Marché, J. D. (2007). Biographical Encyclopedia of Astronomers. Springer.
Malacara, D. (2007). Optical Shop Testing. Hoboken (NJ): John Wiley & Sons.
Myers, R. L. (2006). The Basics of Physics. Westport: Greenwood Publishing Group.
ohare, B. (2009, March 14). File:Fizeau.JPG. Retrieved from http://en.wikipedia.org: http://en.wikipedia.org/wiki/File:Fizeau.JPG
Serway, R. A., & Jewett, J. W. (2007). Physics for Scientists and Engineers. Stamford: Cengage Learning.
