Refractive Index

When a biconvex lens is dipped in a liquid and behaves like a plane sheet of paper, it implies that the lens no longer bends light, meaning there is no refraction at the lens-liquid interface. This condition occurs when the refractive index of the liquid is the same as that of the glass.
For a lens to behave like a plane sheet of paper, the refractive index of the liquid must be equal to the refractive index of the glass.
In this case, the refractive index of glass is 1.5, so the refractive index of the liquid must also be 1.5.