Question

Define the following terms for a lens:
(a) optical centre
(b) principal axis
(c) principal focus
(d) focal length

Answer 1

(a) The optical centre of a lens is the point on its principal axis through which a ray of light can pass without deviation.

(b) The line joining the centres of the two spheres C₁-C₂ of which the lenses form a part is called the principal axis of the lens.



(c) The point where a beam of light, parallel to the principal axis, converges after passing through a converging lens or appears to converge in case of a diverging lens is called the principal focus.


(d) The distance between the optical centre and the focus of a lens is called its focal length.