This paper describes a method of finding the refractive index of liquids using a thin plano-convex lens and a transmission diffraction grating. Light from a low power helium- neon laser is diffracted by a transmission diffraction grating and the diffracted light is received by the thin plano-convex lens L and focuses it at the image distance v. A formula is derived to calculate the refractive index of experimental liquid at a particular
temperature.