Speed of Light

Light travels with a very high but finite speed. In vacuum this speed is denoted $c$, with value $c=3\times10^{8}\ \text{m s}^{-1}$. Several scientists attempted to measure this speed; the earliest successful terrestrial measurement was made by the French scientist Hippolyte Fizeau (1819-1896), using a rotating toothed wheel, described in the next section. Later scientists such as Foucault (1819-1868) and Michelson (1852-1931) refined the measurement further and also introduced different transparent media (glass, water, even evacuated glass tubes) into the light's path, establishing that light always travels more slowly in a material medium than in vacuum, while its speed in vacuum and in ordinary air are nearly indistinguishable. This section builds towards the definition of refr …