Consider a typical metallic conductor such as Copper, whose electrical conductivity at room temperature is about (Wikipedia contributors 2012). Copper, therefore, acts as a good conductor for all electromagnetic waves of frequency below about . The skin-depth in Copper for such waves is thus
The conductivity of sea-water is only about (Wikipedia contributors 2012). However, this is still sufficiently high for sea-water to act as a good conductor for all radio frequency electromagnetic waves (i.e., GHz). The skin-depth at 1MHz ( m) is about m, whereas that at 1kHz ( km) is still only about 7m. This obviously poses quite severe restrictions for radio communication with submerged submarines. Either the submarines have to come quite close to the surface to communicate (which is dangerous), or the communication must be performed with extremely low frequency (ELF) waves (i.e., Hz). Unfortunately, such waves have very large wavelengths ( ), which means that they can only be efficiently generated by gigantic antennas.
According to Equation (868), the phase of the magnetic component of an electromagnetic wave propagating through a good conductor lags that of the electric component by radians. It follows that the mean energy flux into the conductor takes the form (see Appendix C)
According to Equation (870), the impedance of a good conductor is far less than that of a vacuum (i.e., ). This implies that the ratio of the magnetic to the electric components of an electromagnetic wave propagating through a good conductor is far larger than that of a wave propagating through a vacuum.
Suppose that the region is a vacuum, and the region is occupied by a good conductor of conductivity . Consider a linearly polarized plane wave normally incident on the interface. Let the wave electric and magnetic fields in the vacuum region take the form of the incident and reflected waves specified in Equations (812) and (813). The wave electric and magnetic fields in the conductor are written
According to the previous analysis, a good conductor reflects a normally incident electromagnetic wave with a phase shift of almost radians (i.e., ). The coefficient of reflection is just less than unity, indicating that, while most of the incident energy is reflected by the conductor, a small fraction of it is absorbed.
High quality metallic mirrors are generally coated in Silver, whose conductivity is (Wikipedia contributors 2012). It follows, from Equation (882), that at optical frequencies ( ) the coefficient of reflection of a silvered mirror is . This implies that about of the light incident on the mirror is absorbed, rather than being reflected. This rather severe light loss can be problematic in instruments, such as astronomical telescopes, that are used to view faint objects.