Impedance of free space. In electromagnetism, the impedance of free space, , is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, 
 
where  is the electric field strength, and  is the magnetic field strength. Its presently accepted value is
 ,
where Ω is the ohm, the SI unit of electrical resistance. The impedance of free space (that is, the wave impedance of a plane wave in free space) is equal to the product of the vacuum permeability  and the speed of light in vacuum . Before 2019, the values of both these constants were taken to be exact (they were given in the definitions of the ampere and the metre respectively), and the value of the impedance of free space was therefore likewise taken to be exact. However, with the revision of the SI that came into force on 20 May 2019, the impedance of free space as expressed with an SI unit is subject to experimental measurement because only the speed of light in vacuum  retains an exactly defined value.

Terminology 
The analogous quantity for a plane wave travelling through a dielectric medium is called the intrinsic impedance of the medium and designated  (eta). Hence  is sometimes referred to as the intrinsic impedance of free space, and given the symbol . It has numerous other synonyms, including:

 wave impedance of free space,
 the vacuum impedance,
 intrinsic impedance of vacuum,
 characteristic impedance of vacuum,
 wave resistance of free space.

Relation to other constants 

From the above definition, and the plane wave solution to Maxwell's equations,

where
  H/m is the magnetic constant, also known as the permeability of free space,
  F/m is the electric constant, also known as the permittivity of free space,
  is the speed of light in free space,

The reciprocal of  is sometimes referred to as the admittance of free space and represented by the symbol .

Historical exact value 

Between 1948 and 2019, the SI unit the ampere was defined by choosing the numerical value of  to be exactly . Similarly, since 1983 the SI metre has been defined relative to the second  by choosing the value of  to be . Consequently, until the 2019 revision,
  exactly,
or
  exactly,
or
 
This chain of dependencies changed when the ampere was redefined on 20 May 2019.

Approximation as 120π ohms 

It is very common in textbooks and papers written before about 1990 to substitute the approximate value 120 ohms for . This is equivalent to taking the speed of light  to be precisely  in conjunction with the then-current definition of  as . For example, Cheng 1989 states that the radiation resistance of a Hertzian dipole is
   (result in ohms; not exact).

This practice may be recognized from the resulting discrepancy in the units of the given formula. Consideration of the units, or more formally dimensional analysis, may be used to restore the formula to a more exact form, in this case to

See also 
 Electromagnetic wave equation
 Mathematical descriptions of the electromagnetic field
 Near and far field
 Sinusoidal plane-wave solutions of the electromagnetic wave equation
 Space cloth
 Vacuum
 Wave impedance

References and notes

Further reading 
 

Electromagnetism
Physical constants