The discrepancies between observed and theoretically calculated period ratios of the double mode, classical Cepheids and other groups of Cepheid type variables have played a prominent role in investigations of the important physical characteristics of these stars during the last twenty years. Today, there is growing consensus that the drastic increases in astrophysical opacities proposed by Simon (1982) provide the correct solution of all the period ratio problems (e.g. Petersen, 1989; Iglesias and Rogers, 1992; Moskalik et al., 1991).

It seems at first strange that the optimal opacity change (see Andreasen, 1988) can produce both the decrease in period ratio for double mode classical Cepheids and the increased period ratio in models of δ Scuti stars. Here we use ĸ-effect-functions (Refsdal and Stabell, 1972) to provide both a detailed description of the effects of opacity changes and an improved and more direct understanding of these effects.