Opacity

The solar light emitted by hydrogen can not radiate directly to the observer. There are so many hydrogen atoms around that any photon will be absorbed and re-emitted many times before it can leave the Sun. We call this absorption effect opacity. It causes a central depression instead of a peak in the line profile: all spectral lines of hydrogen are self-reversed. The strongest absorption is found in the alpha line of the Lyman series. The depths of the self-reversal is an indicator of how much neutral hydrogen in its ground state is sitting along the line-of-sight.