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 9