How is it convenient to obtain image by lens practically?

Explanation

To find the image of an object by a lens, we cai in principle, take any two rays emanating fron a point on an object, trace their paths using thi laws of refraction and find the point where the refracted rays meet or appear to meet). Ir 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 ray of light passing through the first principal focus (for a convex lens) or appearing to meet at it (for a concave lens) emerges parallel to the principal axis after refraction.
It must be remembered that each point on an object gives out infinite number of rays. All these rays will pass through the same image point after refraction at the lens.