What is a racemic mixture and why is it not optically active?

A racemic mixture contains a 50-50 mixture of two optical isomers. Optical isomers are identical in molecular structure but rotate the plane of polarized light in different directions (one rotates clockwise, the other anticlockwise).As a racemic mixture contains an equal amount of both isomers, they cancel out each other's effect on light and the mixture is notoptically active as a result. An example of this is the formation of the two isomers of 2-hydroxypropanoic acid from ethanal (which can be shows on a whiteboard).