Use the Medium Properties node to specify the refractive index of the medium.

The default Refractive index n (dimensionless) uses values From material. For User defined enter a value or expression. Only ray propagation in isotropic media can be modeled with the Geometrical Optics interface.

If the Intensity computation is set to Compute intensity, Compute intensity and power, Compute intensity in graded media, or Compute intensity and power in graded media under the physics interface Intensity Computation section, specify the Refractive index, imaginary part k (dimensionless). By default the Refractive index, imaginary part k uses values From material. For User defined enter a value or expression. The convention followed by the Geometrical Optics interface is that an the imaginary part of the refractive index is negative in absorbing media. A positive value indicates a gain medium, meaning that the ray intensity will typically increase along the ray trajectory.

The refractive index can be a function of variables that are defined on the rays or on the domains. However, to make the refractive index a function of a ray property such as frequency or free-space wavelength, the ray variable must be enclosed in the noenv() operator. For example, to make the refractive index a linear function of the free-space wavelength, enter an expression such as 1.5-0.01*noenv(gop.lambda0/1[um]).