At a Material Discontinuity between two different media, the wave vector is reinitialized using Snell’s law. First, the angle of incidence θi is computed:

where ni is a unit vector in the direction of the incident ray and ns is a unit vector normal to the material discontinuity.

At a boundary between two isotropic, non-absorbing media, the refracted ray propagates in the direction nt given by the following relations:

where the ray propagates from the medium with refractive index n1 into the medium with refractive index n2. Because the two media are non-absorbing, the quantities n1, n2, θi, and θt are real-valued. If the media are weakly absorbing, the real parts of n1 and n2, are used. If the media are strongly absorbing, several corrections to Snell’s law and the Fresnel equations must be made, as discussed in Refraction in Strongly Absorbing Media.

If the ray intensity is computed, the release of secondary rays may be suppressed when the intensity of the reflected ray is less than the user-defined Threshold intensity Ith (SI unit: W/m2).