A hollow double convex lens is made of very thin transparent material. It can be filled with air or either of the two liquids L₁ or L₂ having refractive indices n₁ and n₂ respectively (n₂ > n₁ > 1). The lens will diverge a parallel beam of light if it is filled with

L₁ inside and immersed in L₂

A convex lens behave as a diverging lens if refractive index of lens is less than outside medium.For diverging the parallel beam, the liquid (medium) inside the lens should have less refractive index than the outer medium

Hence, n₂ > n₁

∴ Liquid inside is L₁ and liquid outside is L₂.