Question

Why free space offer impedence to electromagnetic waves?

Answer 1

The impedance of free space (more correctly, the wave impedance of a plane wave in free space) equals the product of the vacuum permeability μ0 and the speed of light in vacuum c0. Since the values of these constants are exact (they are given in the definitions of the ampere and the metre respectively), the value of the impedance of free space is likewise exact. However, with the redefinition of the SI base units which are going into force on May 20, 2019, this value is subject to experimental measurement.