A thin convex lens is made of two materials with refractive indices n₁ and n₂, as shown in figure. The radius of curvature of the left and right spherical surfaces are equal. f is the focal length of the lens when n₁ = n₂ = n. The focal length is f + Δf when n₁ = n and n₂ = n + Δn. Assuming Δn ≪ (n − 1) and 1 < n < 2, the correct statement(s) is/are,