Why plane of polarisation of light gets rotated on passing through an optically active substance and not the molecules of optically active solution?

Optical activity occurs in solutions of chiral molecules such as sucrose (sugar), solids with rotated crystal planes such as quartz, and spin-polarized gases of atoms or molecules.
a monochromatic linearly polarized light beam can be considered as a superposition of two circularly polarized electromagnetic waves that are propagating in the same direction with the same frequency but the opposite sense of rotation. The plane of polarization of the resulting linearly polarized wave thus prepared can be changed (rotated) by applying a phase shift between its two circularly polarized components. With the help of this concept we can explain the phenomenon of optical rotation:
As chiral molecules interact slightly differently with the two circularly polarized components of a linearly polarized light beam. This is true both for absorption and refraction. Left- and right hand circularly polarized light beams also have slightly different refractive indices in a chiral medium. This means that even if they are not absorbed they travel at different speeds through the medium. Therefore, this causes a phase shift between the two circularly polarized components which increases proportional to the path length that the light travels through the chiral medium. This phase shift manifests itself as a rotation of the plane of polarization of the resultant linearly polarized light beam - optical rotation.