<p align="right">Last Update: <font color="#4f81bd">November 24, 2024</font></p> ## BIG IDEAS - A [[Physical constant|physical constant]] that is the speed of light in a vacuum (empty space) . - The symbol for the speed of light is c. - c is equal to 299,792,458 meters per second. - c helps to define the [[Meter|meter]], [[Kilogram|kilogram]] and kelvin. > [!NOTE] Historical Term > The speed of light is a historical term. Light is a portion of the electromagnetic spectrum. However, it has been determined that all electromagnetic radiation travels in a vacuum at this maximum speed. ### Determining the Speed of Light - **Ole Rømer (1676)** - **Experiment**: Observed the timing of Jupiter's moons. - **Contribution**: First quantitative estimate of the speed of light by noting discrepancies in the orbit of Io, one of Jupiter's moons. - **Result**: Estimated speed was about 220,000 km/s. - **Hippolyte Fizeau (1849)** - **Experiment**: Used a rotating toothed wheel and a beam of light reflected off a distant mirror. - **Contribution**: First terrestrial measurement of the speed of light. - **Result**: Calculated speed was approximately 313,000 km/s. - **Léon Foucault (1862)** - **Experiment**: Improved Fizeau's method using a rotating mirror. - **Contribution**: More accurate measurement and established the speed of light in water. - **Result**: Determined speed to be about 298,000 km/s. - **Albert A. Michelson (1879-1931)** - **Experiment**: Series of experiments using refined rotating mirror apparatus and various distances. - **Contribution**: Improved accuracy of the speed of light measurements. - **Result**: Final measurement in 1931 was 299,796 km/s. - **International Committee for Weights and Measures (1983)** - **Event**: Redefined the meter based on the speed of light. - **Contribution**: Speed of light was fixed as exactly 299,792,458 meters per second, providing a precise standard for scientific measurements. ### Symbol The lowercase $c$ (from Latin _celeritas_, meaning speed) is currently used for the speed of light. [[Albert Einstein]] used the letter $L$ in his 1905 paper on [[Special Relativity]] but changed to using $c$ in lectures and papers by 1907. ### Formula The speed of light in a medium is given by: $v=\frac{c}{n}$ where: - $v$ is the speed of light in the medium, - $c$ is the speed of light in a vacuum (approximately $3 \times 10^8$ m/s), - $n$ is the refractive index of the medium. ## For Further Reading - Perkowitz, S. (1996). _The Speed of Light: Constancy + Cosmos_. Basic Books. - Webb, S. (1999). _Measuring the Universe: The Cosmological Distance Ladder_. Springer London. - Falk, S. (2020). _The Light Ages: The Surprising Story of Medieval Science_. W. W. Norton & Company. - Linton, C. M. (2004). _From Eudoxus to Einstein: A History of Mathematical Astronomy_. Cambridge University Press. - Galison, P. (2003). _Einstein's Clocks, Poincaré's Maps: Empires of Time_. W.W. Norton & Company. ### Related Topics --- [[Home|Home]] | [[Oscillations]] | [[Electromagnetic radiation]] | [[SI units]] | [[Physical constant|Physical constants]]