Question

(a)For a convex lens, what is the distance of the real image from the optical centre?
(b) What is the focal length of a concave lens?
(A) a = positive, b = positive
(B) a = negative, b = negative
(C) a = positive, b = negative
(D) a = negative, b = positive

Answer 1

Hint
In this problem, we have to know about the functions of the concave lens and the convex lens. And how the concave lens focuses the object to the eye retina and how the convex lens focuses the object to the eye retina. By using the ray diagram, the solution can be determined easily.

Complete step by step solution
(a) Convex lens
The convex lenses are used with eyeglasses for the correction of farsightedness, farsightedness means that the person can see the far object clearly than the close objects. If the person can see the close object, the object is focused next to the eye retina. So, the convex lens is used to focus the object to the eye retina. So, the distance of the real image from the optical centre is positive, because the object is focused next to the eye retina.
(b) Concave lens
The concave lenses are used with eyeglasses for the correction of near-sightedness, near-sightedness means that the person can see the close object more clearly than the far objects. If the person can see the far object, the object is focused in front of the eye retina. So, the concave lens is used to focus the object to the eye retina. So, the distance of the real image from the optical centre is negative, because the object is focused in front of the eye retina.
Hence, the option (C) is the correct answer.

Note
In a convex lens we have to reduce the distance to focus the object to the retina, that means the distance is long, so the answer is positive for a convex lens. In a concave lens we have to increase the distance to focus the object to the retina, that means the distance is small, so the answer is negative for the concave lens.