A Most Curious Wave
The waves we discussed in Chapters 17 and 18 require a medium (some material) . through which or along which to travel. We had waves traveling along a string, through Earth, and through the air. However, an electromagnetic wave (let’s use the term light wave or light) is curiously different in that it requires no medium for its travel. It can, indeed, travel through a medium such as air or glass, but it can also travel through the vacuum of space between a star and us.
Once the special theory of relativity became accepted, long after Einstein pubIished it in 1905, the speed of light waves was realized to be special. One reason is that light has the same speed regardless of the frame of reference .from which it is measured. If you send a bear of light along an axis and ask several observers to measure its speed while they move at different speeds along that axis, either in the direction of the light or opposite it, they will all measure the same speed for the light. This result is an amazing one and quite different from what would have been found if those observers had measured the speed of any other type of wave; for other waves, the speed of the observers relative to the wave would have affected their measurements.
measurements. The meter has now been defined so that the speed of light (any electromagnetic wave) in vacuum has the exact value
c = 299792458 tul«,