Here we are concerned only with thin lenses i.e. the thickness of the lens is negligible.
Let us learn the terminology used in the case of lenses. Each curved surface of a lens is part of a sphere. The centre of the sphere which contains the part of the curved surface is called centre of curvature. It is denoted by a letter ‘C’. If a lens contains two curved surfaces then their centres of curvature are denoted as C1 and C2 .The distance between the centre of curvature and curved surface is called radius of curvature (R). Radii of curvature are represented by R1 and R2 respectively. Let us consider a double convex lens as shown in fig.- 7.
The line joining the points C1 and C2 is called principal axis. The midpoint of a thin lens is called optic centre of lens (P).
A parallel beam of light incident on a lens converges to a point as shown in fig.- 8(a) or seems to emanate from a point on the principal axis as shown in fig.- 8(b). The point of convergence (or) the point from which rays seem to emanate is called focal point or focus (F). Every lens has two focal points. The distance between the focal point and optic centre is called the focal length of lens denoted by ‘f’
For drawing ray diagrams related to lenses we represent convex lens with a symbol R and concave lens with . See figures 8(c) and 8(d).
• How does the lens form an image?