Question

Explain the following with the help of a ray diagram, in relation to the two main types of spherical lenses:

(a) principal focus
(b) focal length

Answer 1

Spherical lenses are mainly of two types: convex lens and concave lens.

(a) Principal focus:
Convex lens: In a convex lens, the parallel rays of light passing through the lens actually meet at a point in front of the lens known as focus. In the following figure, F is the focus.


Concave lens: In a concave lens, the rays of light passing through the lens diverge and when these diverging rays are produced backward, they appear to meet at a point behind the lens. The point is called the focus of concave lens. In the following figure, F is the focus.



(b) Focal length
Convex lens:
Focal length of a convex lens is the distance between the focus F and the optical centre O. In the figure, OF is the focal length. The focal length is on the right side in a convex lens.
Concave lens:
Focal length of a concave lens is the distance between the focus F and the optical centre O. In the figure, FO is the focal length. The focal length is on the left side in a convex lens.