It is worth mentioning that the two foci, F and F¢, of a double convex
or concave lens are equidistant from the optical centre The focus on the
side of the (original) source of light is called the first focal point, whereas
the other is called the second focal point 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