# Lens Introduction | Terms Used | Physics Grade XI

### Lenses

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# Lens Introduction | Terms Used in Lens

Lens: A lens is an object, usually made of glass, bounded by one or to spherical surfaces. There are two types of lenses and they are;

1. Convex or converging lens
2. Concave or diverging lens

There are different type of convex and concave lenses as shown below:-

Term Used:

1. Center of curvature: The center of the sphere of which lens form a part is called center of curvature. A lens has two center of curvature C1 and C2.
2. Radius of curvature: The radius of the sphere of which lens form a part is called center of curvature. A lens has two radius of curvature R1 and R2.
3. Principle axis: It is the line joining the two centers of curvature of two refracting surfaces of lens.
4. Optical center: The geometrical center of the lens through which light never bends after passing through it is called optical center. It is denoted by C.
5. Principal focus: The point on the principle focus where all the parallel beam of light after refraction is passed or appear to pass through it. It is denoted by F.
6. Focal length: The distance between principle focus and the optical center of the lens is called focal length. It is denoted by f. it is positive for convex lens whereas negative for concave lens.
7. Focal plane: The plane passing through the focus and perpendicular to the principal axis of a lens is called the focal plane.

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