Kerosene, Ethanol, Ethyl Alcohol, Benzene etc. are organic compounds. The contain mainly Carbon and Hydrogen. The molecules are long chains. They space occupied by the molecules is relatively larger than water.. The number of atoms in a molecule is more than that of water. But the mass of atoms is less than oxygen. Hence their volume density is lower than water.
But water is an inorganic compound. Water has a density of 1 gm/cc. Water has a refractive index of 1.33.
These organic compounds have volume density around 0.80 gm/cm^3. But their optical density or refractive index is around 1.4 or 1.5.
The light wave (electromagnetic wave) enters a material and excites electrons. They absorb energy and vibrate. When their natural frequency and the frequency of the radiation match, then electrons absorb that energy. If they do not, then the absorbed energy is released by the electrons. That travels again through the empty space in the atoms and inter atomic space as well as the intermolecular space. Thus light travels through material by absorptions and re-emissions.
The amount of delay involved in this process of absorption and reemission causes slowing down of the transmission of light through the material. The ratio of the velocity of light in vacuum and that in the material gives the refractive index of the material.
If the material contains atoms with the valency/conduction electrons in such a state that they absorb and release energy with more delay, then the refractive index is higher. Then the light ray bends more. The wavelength of the light ray increases in the medium. Also the colour of the radiation changes.
Thus the way the electrons in carbon and hydrogen in a transparent organic compound like benzene, ethanol, kerosene respond to the incoming radiation energy decides the refractive index.