Turpentine is optically denser than water. It means 

Optical density of turpentine oil (kerosene oil) is more than that of water, even though water is more mass dense than oil.Optical density is a function of number or atoms and NOT the weight of a substance. Since kerosene is a complex carbon compound, it has more number of atoms per molecule than in water. So when light enters kerosene, it hinders it better as compared to water. hence in denser medium velocity decreases.