Photonics
- Refractive index
The (absolute) refractive index of a medium n (dimensionless) is an inherently important property of geometric and physical optics defined as the ratio of the luminal speed in vacuum c0 to that in the medium c:
where ε is the permittivity and εr the relative permittivity of the medium, likewise μ is the permeability and μr are the relative permmeability 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 consequnce of the definition, the speed of light in matter is
for special case of vacuum; ε = ε0 and μ = μ0,
- 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):
Read more about this topic: Constitutive Equation