Refractive Index
The refractive index is a complex-valued material property. Its real part indicates the speed of light within the material; its imaginary part describes how quickly a beam will attenuate (that is, decrease in intensity) as it passes through the material.
The refractive index and other properties of domains included in the Geometrical Optics interface can be set using the Medium Properties node. It is also possible to specify the refractive index of the void surrounding a Geometrical Optics model.
Absolute vs Relative Index
When looking at refractive index data for a given material, it is essential to know whether the refractive index is absolute or relative:
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An absolute refractive index is defined with respect to vacuum. That is, if the absolute index of a material is 1.5, the speed of light in that material is c/1.5, where c = 299,792,458 m/s is the speed of light in a vacuum.
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A relative refractive index is defined with respect to some other material, most often air. If the relative index of a material is 1.5, then the speed of light in that material is c/1.5, where c is the speed of light in air.
Furthermore, the speed of light in air is a function of temperature and pressure. Typically, when defining the relative refractive index, the air is at a reference temperature of 1 atm and a reference temperature of 20°C, although some glass catalogs use a different reference temperature such as 15°C.
The settings for the Medium Properties always indicate whether the specified refractive index is absolute or relative; and if the given refractive index is relative, a reference temperature and reference pressure are always shown.
During results processing, the variable gop.nref is always the absolute refractive index, regardless of whether the absolute or relative index was specified.
Exterior and Unmeshed domains
In the physics interface settings Material Properties of Exterior and Unmeshed Domains section, the Optical dispersion model is specified. This determines the Refractive index of exterior domains, real part. This refractive index is used in any domains outside of the selection for the Geometrical Optics interface, as well as the void domain outside the geometry. It is a spatially uniform, scalar-valued quantity. The refractive index outside the domain selection cannot depend on field variables such as temperature or pressure and cannot be a graded-index medium. For domains with temperature-dependent or spatially nonuniform refractive indices, use the Medium Properties node instead.