  Light rays from a multiply imaged quasar usually sample different path
lengths across the deflector. Extinction in the lensing galaxy may thus lead to
a differential obscuration and reddening between the observed macro-lensed QSO
images. These effects naturally depend on the precise shape of the extinction
law and on the redshift of the lens. By means of numerical Monte-Carlo
simulations, using a least-squares fitting method and assuming an extinction
law similar to that observed in the Galaxy, we show how accurate photometric
observations of multiply imaged quasars obtained in several spectral bands
could lead to the estimate of the lens redshift, irrespective of the visibility
of the deflector. Observational requirements necessary to apply this method to
real cases are thoroughly discussed. If extinction laws turn out to be too
different from galaxy to galaxy, we find out that more promising observations
should consist in getting high signal-to-noise low resolution spectra of at
least three distinct images of a lensed quasar, over a spectral range as wide
as possible, from which it should be straightforward to extract the precise
shape of the redshifted extinction law. Very high signal-to-noise, low spectral
resolution, VLT observations of H1413+117 and MG 0414+0534 should enable one to
derive such a redshifted extinction law.
