# Lens

Chapter-9|Ray Optics and Optical Instrument|NCERT 12th Physics:

Lens:

Lens is a transparent refracting medium bounded by two surfaces, one of which must be spherical.

Lenses are two types-

(I) Convex or converging lens

(II) Concave or diverging lens

Convex Lens: A lens which is thicker at the centre and thinner at its ends is called convex lens. Convex lens are of three types.

Types of Convex Lens:

Concave Lens: A lens which is thinner at the centre and thicker at its ends is called a concave lens.

Types of Concave Lens:

Some Definitions Related to Lenses:

(I) Optical Centre: The centre point of a lens is known as its optical centre.

(IV) Principal Axis: The imaginary line joining the two centres of curvature is called principal axis of lens.

(V) Principal Focus: Lens has two principal foci.

(a) First Principal Focus: It is a point on the principal axis of lens, the rays starting from this point in convex lens or rays directed to this point in concave lens, become parallel to principal Axis after refraction.

(b) Second Principal Focus: It is the point on the principal axis at which the rays coming parallel to the principal Axis converge(convex lens) or passing through it apear to diverge(concave lens) at this point after refraction from the lens.

(VI) Aperture: The effective diameter of the circular outline of a spherical lens is called its aperture.

(VII) Refractive Axis: It is an imaginary Axis at the optical centre perpendicular to the principal Axis which represents the lens.