Optical activity: When the plane of polarization is rotated by plane-polarized light. The optical activity is caused only when the compound is chiral. It can be measured by polarimetry. The combination of two optically active compounds forms an optically inactive compound.
The racemic mixture consists of a mixture of enantiomers. They are chemically identical and their physical properties are also the same but the direction of the plane of polarized light is different. This means enantiomers are optically active.