  The paper analyzes the absorption line profile at 21 cm for the hydrogen atom
in the interstellar medium. The hydrogen atom is treated as a three-level
system illuminated by a powerful light source at neighboring resonances
corresponding to the hyperfine splitting of the ground state and Ly$_\alpha$
transition. The field acting upon the resonances gives rise to physical
processes, which can be explained as interfering pathways between different
transitions. The paper considers particular cases when the 21 cm line profile
is substantially modified by the Ly$_\alpha$ transition. A correction to the
optical depth is introduced as a result of theory. It is shown that the
correction can be considerable and should be taken into account when
determining the column density of hydrogen atoms in the interstellar medium.
The paper also deals with the effects of none-Doppler broadening and frequency
shift.
