A lens is a portion of a transparent glass bound by two spherical surfaces. Lenses are of two types:
Convex Lens or Convergent Lens:- A convex lens is thick in the middle and thin at the edges.
Concave Lens or Divergent Lens:- A concave lens is thin in the middle and thick at the edges.
Some Important Terms Related to Lenses
Optical Centre
The centre point of a lens which lies on the principal axis of the lens is called its optical centre. The optical centre is represented by letter C. See figure given below
Principal Axis
The principal axis of a lens is defined as a straight line passing through the optical centre and the centre of curvature of two surfaces of a lens. In figure given below, the line C1 C2 represents principal axis of both the convex lens and concave lens.
Principal Focus
The principal focus of a lens is a point on its principal axis to which the light rays parallel to the principal axis converge (in case of convex lens) or appear to diverge (in case of concave lens) after passing through it. The principal focus of a lens is represented by letter F as shown in figure given below.
Focal Length
The focal length of a lens is the distance between its optical centre and principal focus. The focal length of a lens is represented by letter f. In figure given above, the distance CF is the focal length of the lens.
Test Your Understanding and Answer These Questions:
- Define spherical lens.
- What are types of spherical lens?
- Define convex lens.
- Define concave lens.
- Define principal axis.
- Define optical centre.
- Why convex lens is called a converging lens?
- Why concave lens is called a diverging lens?
- Give differences between convex lens and concave lens.
- Explain the terms principal focus and focal length with the help of a diagram.