We present a new technique for describing geodesic congruences (for example the trajectory of a gas of photons or particles) crossing a generic light-like shell in vacuum. In particular the change in the direction of the vector field of the congruence has been derived and shown to be in agreement with the results of Barrabes and Hogan thus demonstrating the validity of this technique. For the case of null shells generated by infinitesimal BMS supertranslation type soldering, the expansion and shear tensors of the congruence have also been calculated. We derived the surface equation, for relative motion orthogonal to the congruence after it crosses the shell, proving also that the congruence is always hypersurface orthogonal and the rotation tensor is always zero. This means that the Eikonal approximation can be applied to describe it as a wave on the future, and this has been carried out for a special BMS type soldering. The conditions for focusing, defocusing and formation caustics have been studied.
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