If the solution are equally concentrated the amount of rototation caused by the two isomers is exactly the same , but in opposite directions. When optically active substance are made in the lab, they often occurs as a 50/50 mixture of the two enantiopmers. This is known as a racemic mixture or racemate. It has no effect on plane polarised light.
Enantiomers have :
(a) Identical melting point / boiling point but different refractive indices
(b) Identical melting point / boiling point refractive indices but rotate plane polarised light in opposite directions but to the same extent.
(c) Different refractive indices and rotates plane polarized light in the same direction but to different extends
(d) Different melting/ boiling points but rotate plane of polarised light in different directions but to the same extends.