  We consider conformal defects joining two conformal field theories along a
line. We define two new quantities associated to such defects in terms of
expectation values of the stress tensors and we propose them as measures of the
reflectivity and transmissivity of the defect. Their properties are
investigated and they are computed in a number of examples. We obtain a
complete answer for all defects in the Ising model and between certain pairs of
minimal models. In the case of two conformal field theories with an enhanced
symmetry we restrict ourselves to non-trivial defects that can be obtained by a
coset construction.
