Two transparent media of refractive indices n 1 and n 2 are separated by a spherical…

Two transparent media of refractive indices n₁ and n₂ are separated by a spherical transparent surface. The rays of light incident on the surface get refracted into the medium on the other side. The laws of refraction are valid at each point of the spherical surface. A lens is a transparent optical medium bounded by two surfaces, at least one of which should be spherical. The focal length of a lens is determined by the radii of curvature (R₁ and R₂) of its two surfaces and the refractive index (n) of the medium of the lens with respect to the surrounding medium. Depending on R₁ and R₂, a lens behaves as a diverging or a converging lens. The ability of a lens to diverge or converge a beam of light incident on it defines its power.

An equiconcave lens of focal length f is cut into two identical parts along the dotted line as shown in the figure. The focal length of each part will be: