  Instead of using frequency dependent refractive index, we propose to use the
extinction theorem to describe reflection and transmission of an ultrashort
pulse passing through the boundary. When the duration of the pulse is
comparable with the relaxation time, the results differ significantly from
those given by the traditional method, especially if the carrier frequency is
close to an absorbtion line. We compare the two approaches using the data of
GaAs in the infrared domain.
