In this paper we present and discuss the effects of scattered light echoes
(LE) on the luminosity and spectral appearance of Type Ia Supernovae (SNe).
After introducing the basic concepts of LE spectral synthesis, by means of LE
models and real observations we investigate the deviations from pure SN
spectra, light and colour curves, the signatures that witness the presence of a
LE and the possible inferences on the extinction law. The effects on the
photometric parameters and spectral features are also discussed. In particular,
for the case of circumstellar dust, LEs are found to introduce an apparent
relation between the post-maximum decline rate and the absolute luminosity
which is most likely going to affect the well known Pskowski-Phillips relation.Comment: 15 pages, 14 figures, submitted to MNRAS. Full resolution version at
http://www.eso.org/~fpatat/science/LE/paperII.pd