Racemic tartaric acid and meso - tartaric acid are both optically inactive - why?

Explanation

Racemic tartaric acid is an equimolar mixture of (+) and (-) - tartaric acids. In racemic tartaric acid, therefore, the rotatory power of (+) enantiomer is neutralised by the rotatory power of the (-) enatiomer (external compensation) and for this reason, the racemic tartaric acid is optically inactive. On the other hand, the meso - tartaric acid is optically inactive because it has a plane of symmetry and it is superimposable on its mirror image. In fact, in this case, the optical rotation of one half of the molecule is exactly cancelled by the optical rotation of the order half (internal compensation).