**Various Terms related with Lenses:**

**1) Optical centre : **The point inside a lens on the principal axis, through which light rays pass without changing their path is called the optical centre (O) of the lens.

**2) Centre of curvature : **The centres of the spheres whose parts form the surfaces of a lens are called the centres of curvature of the lens. A lens has two centres of curvature C_{1} and C_{2} for its two spherical surfaces.

**3) Radius of curvature : **The distance of the optical centre from either of the centre of curvatures is termed as the **radius of curvature**.

**4) Principal axis : **The imaginary straight line joining the two centers of curvature and the optical centre **(O)** is called the **principal axis** of the lens.

**5) Focus (P****rincipal focus) (F)**** : **When light rays parallel to the principal axis are incident on a convex lens, they converge at a point on the principal axis. This point is called the principal focus (F) of the convex lens.

Light rays travelling parallel to the principal axis of a concave lens diverge after refraction in such a way that they appear to be coming out of a point on the principal axis. This point is called the principal focus of the concave lens. A lens has two principal foci F_{1} and F_{2}.

**6) Focal length : **The distance between the focus **(F _{1} or F_{2})** and the optical centre (

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