The problem of plane thermal waves in media with linearly varying thermal conductivity kappa in the absence of internal heat sources is rigorously solved in terms of Bessel functions. An exact solution in the presence of an internal heat source arising from optical absorption is also presented. Focus is on the semi-infinite solid. In particular, an inverse calculation procedure is outlined that is able to yield, in a very fast way, a linear kappa profile that reveals the most important aspects of the actual profile.