Eclipses. Speed, reflection and refraction of light

Light travels at a very large but finite speed: in a vacuum about 300 000 km/s (more precisely 299 792 km/s). It was first shown in 1676 by the Danish scientist Ole Rømer from eclipses of Jupiter’s moons. In transparent substances light is slower: about 225 000 km/s in water, 200 000 km/s in glass. Light from the Sun to the Earth (about 150 000 000 km) takes about 8.3 minutes (500 s). At the boundary of two media some light is reflected. The law of reflection: the angle of incidence equals the angle of reflection (α = β), and the incident ray, reflected ray and the perpendicular to the surface lie in one plane; angles are measured from the perpendicular. A smooth surface gives mirror reflection, a rough surface scattered reflection (so we see a sheet of paper from all directions). When light passes from one medium into another and changes direction, this is refraction: it happens because the speed changes. That is why a pencil in water looks bent and a pool looks shallower than it is.