This book will be helpful to students pursuing graduation and post graduation who wish to have deeper knowledge in the topic refractive index and it's application. All types of refractometer working are discussed in detail. It is helpful in detecting diabetes a disease to which masses are prone too. Polymer analysis can be done by refractive index study. There are formulations of refractive index correlation with sonic speed, surface tension, excess volume, thermal expansion coefficient. Ionic liquids, refractive index measurements are also feasible. Mixing rules applied to pure and multi-components mixtures are given in detail. The key focus are the experiments designed which can be performed in laboratory with ease are given in detail for smooth working of laboratory.