If the velocity formula_32, acquired by the particle as the result of the above reflection law, is directed to the interior of the domain formula_20, then the particle will leave the boundary and continue moving in formula_20 until the next collision with formula_18. If the velocity formula_32 is directed towards the outside of formula_20, then the particle remains on formula_18 at the point formula_35 until at some time formula_52 the interaction with the boundary will force the particle to leave it.