Speed of Light Through Different Media

Building on Fizeau's method, later scientists like Foucault (1819-1868) and Michelson (1852-1931) introduced different transparent media -- glass, water, and even evacuated glass tubes -- directly into the path of the light being timed, in order to measure the speed of light not just in air but inside real materials, and even in a true vacuum. Their measurements established two key facts used throughout the rest of the chapter: light travels with a lesser speed in any material medium than it does in vacuum, and the speed of light in vacuum and in ordinary air are almost exactly the same (air's refractive index is only 1.0003, barely different from vacu …