To find the image of an object by a lens, we can, in principle, take any
two rays emanating from a point on an object; trace their paths using the
laws of refraction and find the point where the refracted rays meet (or
appear to meet) In practice, however, it is convenient to choose any two
of the following rays:
(i)
A ray emanating from the object parallel to the principal axis of the
lens after refraction passes through the second principal focus F¢ (in
a convex lens) or appears to diverge (in a concave lens) from the first
principal focus F (ii) A ray of light, passing through the optical
centre of the lens, emerges without any
deviation after refraction (iii) (a) A ray of light passing through the first
principal focus of a convex lens [Fig