Let G_n, N∞N, denote the set of gaps of the Hill operator. We solve the following problems: 1) find the effective masses M_n^±, 2) compare the effective mass M_n^± with the length of the gap G_n, and with the height of the corresponding slit on the quasimomentum plane (both with fixed number n and their sums), 3) consider the problems 1), 2) for more general cases (the Dirac operator with periodic coefficients, the Schrödinger operator with a limit periodic potential). To obtain 1)-3) we use a conformal mapping corresponding to the quasimomentum of the Hill operator or the Dirac operator.