An **optical medium** is material through which "electromagnetic waves propagate. It is a form of "transmission medium. The "permittivity and "permeability of the medium define how electromagnetic waves propagate in it. The medium has an *"intrinsic impedance*, given by

where and are the "electric field and "magnetic field, respectively. In a region with no "electrical conductivity, the expression simplifies to:

For example, in "free space the intrinsic impedance is called the "characteristic impedance of vacuum, denoted *Z*_{0}, and

Waves propagate through a medium with velocity , where is the "frequency and is the "wavelength of the electromagnetic waves. This equation also may be put in the form

where is the "angular frequency of the wave and is the "wavenumber of the wave. In "electrical engineering, the symbol , called the *"phase constant*, is often used instead of .

The propagation velocity of electromagnetic waves in "free space, an idealized standard reference state (like "absolute zero for temperature), is conventionally denoted by *c*_{0}:^{[1]}

- where is the "electric constant and is the "magnetic constant.

For a general introduction, see Serway^{[2]} For a discussion of man-made media, see Joannopoulus.^{[3]} Types of optical mediums

1 Homogenous medium 2 Heterogeneous medium 3 Transparent medium 4 Translucent medium 5 Opaque body

**^**With "ISO 31-5, "NIST and the "BIPM have adopted the notation*c*_{0}.**^**Raymond Serway & Jewett J (2003).*Physics for scientists and engineers*(6th ed.). Belmont CA: Thomson-Brooks/Cole. "ISBN "0-534-40842-7.**^**John D Joannopouluos; Johnson SG; Winn JN; Meade RD (2008).*Photonic crystals : molding the flow of light*(2nd ed.). Princeton NJ: Princeton University Press. "ISBN "978-0-691-12456-8.