A magician during a show makes a glass lens with n = 1.47 disppear in a trough of liquid:-What is the refractive index of the liquid?

A magician during a show makes a glass lens with n = 1.47 disppear in a trough of liquid:-Could the liquid be water?

What will be refractive index n_12 ? Refractive index, n_12 =

If the focal length of a double convex lens is 12 cm and radii of curvatures of faces are 10 cm and 15 cm respectively, what is the refractive index of the lens?

When a hydrometer is dipped n a liquid, the reading is found to be 1.25. What is the density of the liquid?