Question

Meso compounds are optically ___.

Answer 1

Meso compounds contain chiral centers ($s{p}^3$ hybridized tetrahedral atoms bonded to 4 different groups) but are achiral overall becausethey have a plane of symmetry. As in, a meso compound should have at least 2 chiral $s{p}^3$ hybridized atoms (Nitrogen atom of a tertiary amine is an exception) such that there is at least one internal plane that splits the molecule into two mirror images.
Since tetrahedral centers are involved, the molecule cannot be planar.

Any compound with even one element of symmetry is achiral – as in, it will not rotate plane polarized light! Hence, meso compounds are optically inactive.