The paper describes a simple theoretical model for explaining optical activity. The model consists of two anisotropic atoms, each having polarizabilityα. It is shown that when there is an assembly of such models, the refractive indices for right and left circular light are different, which means that the medium possesses optical rotatory power. By takingαto be complex in the vicinity of an optically active absorption band, the main features of Cotton effect or circular dichroism have also been explained.