Page 2 - LN

IMPORTANT TERMS IN LENS
 Optical centre: The centre of the lens is known as ‘optical centre’. It is denoted by ‘O’.
 Principal axis: It is the line joining the centers of two spheres from which the lens is
formed.
 Focus: It is the point through which parallel rays converge or diverge.
 Focal length: It is the distance between the optical centre and the focus of a lens.

RULES OF IMAGE FORMATION BY LENS:
 When the incident ray is parallel to the principal axis, it passes through the principal focus of
other side of lens after refraction.
 When incident ray passes through the optical centre, it emerges without any deviation after
refraction.
 When incident ray passes through the principal focus, it emerges parallel to the principal axis
after refraction.

CONVEX LENS:
The lens which is thicker at the centre but thinner at the edges is known as convex lens.
When a parallel beam of light falls on a convex lens, it converge the light rays at its focus. It is
also known as converging lens.

Note: Convex lens behaves as a concave mirror.