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 speed of sound c1 into the medium with speed of sound c2.

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). Similarly, when ray power is computed, a Threshold power Qth (SI unit: W) can be specified.