Speed of light, laws of reflection and refraction

Light seems to travel instantly, but its speed is finite. It was first estimated in 1676 by the Dane Ole Roemer, who observed how the time between eclipses of Jupiter’s moon Io changes along the Earth’s orbit: when the Earth moves away from Jupiter the eclipse is late because the light covers a longer path; c = D/t. The result based on his observations (about 2.2·10⁸ m/s) was smaller than today’s value, but it proved that the speed of light is finite. In 1849 the Frenchman Fizeau measured it on Earth: light passes between the teeth of a toothed wheel, is reflected from a distant mirror and, when the wheel spins at a certain frequency, passes through the next gap; c = 4lNν (l – distance from wheel to mirror, N – number of teeth, ν – rotation frequency). Later Foucault used rotating mirrors (1862) and Michelson an improved method (in the 1920s). The present value in vacuum is c = 299 792 458 m/s, and since 1983 the metre is defined through this speed; in problems we take c = 3·10⁸ m/s. No body can move faster than the speed of light in vacuum.