Question

Petrol is less denser than water. Inspite of this it has greater optical density than water. Why?

Answer 1

Optical density of a material determines the ability of the material to refract light. It is not the same as mass density of the medium, which determines the heaviness of the material. That is why petrol inspite of being less denser in terms of mass than water, it is optically more denser than water.