permeability and μr are the relative permeability of the medium. The vacuum permittivity is ε0 and vacuum permeability is μ0. In general, n (also εr) are complex numbers. The relative refractive index is defined as the ratio of the two refractive indices. Absolute is for on material, relative applies to every possible pair of interfaces; Speed of light in matter As a consequence of the definition, the speed of light in matter is for special case of vacuum; and , Piezooptic effect The piezooptic effect relates the stresses in solids σ to the dielectric impermeability a, which are coupled by a fourth-rank tensor called the piezooptic coefficient Π (units K−1): Transport phenomena Definitions Definitive laws There are several laws which describe the transport of matter, or properties of it, in an almost identical way. In every case, in words they read: Flux (density) is proportional to a gradient, the constant of proportionality is the characteristic of the material. In general the constant must be replaced by a 2nd rank tensor, to account for directional dependences of the material. See also Governing equation Principle of material objectivity Rheology Notes References Elasticity (physics) Equations of physics Electric and magnetic fields in matter