Draw a figure explaining various terms related to a lens.

Explanation

Concepts related to lens:

(i) Centre of curvature (C): The centres of spheres whose parts form surfaces of the lenses are called centres of curvatures of the lenses. A lens with both surfaces spherical, has two centres of curvature `c_(1) " and " c_(2)`
(ii) Radius of curvature (R): The radii `(R_(1) " and R_(2))` of the lens.
(iii) Principal axis: The imaginary line passing through both centres of curvature is called the principal axis of the lens.
(iv) Optical centre (0): The point inside a lens on the principal axis, through which light rays pass without changing their path is called the optical centre of a lens. In figure 7.14, rays` P_(1) Q_(1)`, `P_(2)Q_(2)` passing through O are going along a straight line. Thus, O is the optical centre of the lens.

(v) Principal focus (F): When light rays parallel to the principal axis are incident on a convex lens, they converge to a point on the principal axis. This point is called the principal focus of the lens. As shown in figure 7.15a `F_(1)" and "F_(2)` are the principal foci of the convex lens.

Light rays parallel to the principal axis falling on a convex lens come together , i.e. get focused at a point on the principal axis . So , this type of lens is called a converging lens.
Rays travelling parallel to the principal axis of a concave lens diverge after refraction m 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. As shown in figure 7.15 (b) `F_(1)" and "F_(2)` are the principal foci of the concave lens.
Light rays parallel to the principal axis falling on a concave lens go away from one another (diverge) after refraction. So, this type of lens is called a diverging lens.
(vi) Focal length (L): The distance between the optical centre and principal focus of a lens is called its focal length.