Question

Define the following terms applied to a lens:
(i) Principal axis (ii) Optical centre (iii) Focal length

Answer 1

Ans. PRINCIPAL AXIS
The line passing through the centre of the lens is called principal focus.
OPTICAL CENTRE
A point on the principal axis at the centre length lens is called optical centre.

FOCAL LENGTH
The distance between the optical centre and the principal focus is known as focal length.
0.27. What is meant by the principal focus of a
(a) Convex lens
(b) Concave tens. Illustrate your answer with ray diagrams.
Ans. (a) In case of a convex lens:
The light rays travelling parallel to the principal axis of a convex lens after refraction meet at a point on the principal axis called principal focus or focal point.