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The impedance of free space, ''Z''0, is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, ''Z''0 = |E|/|H|, where |E| is the electric field strength and |H| magnetic field strength. It has an exact irrational value, given approximately as 376.73031... ohms.〔 〕 The impedance of free space (more correctly, the wave-impedance of a plane wave ''in'' free space) equals the product of the vacuum permeability or magnetic constant ''μ''0 and the speed of light in vacuum ''c''0. Since the numerical values of the magnetic constant and of the speed of light are fixed by the definitions of the ampere and the metre respectively, the exact value of the impedance of free space is likewise fixed by definition relative to the S.I. base units. ==Terminology== The analogous quantity for a plane wave travelling through a dielectric medium is called the ''intrinsic impedance'' of the medium, and designated η (eta). Hence ''Z''0 is sometimes referred to as the ''intrinsic impedance of free space'', and given the symbol η0.〔 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''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「impedance of free space」の詳細全文を読む スポンサード リンク
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