Question

The near point of a long-sighted person is $40\,cm$ from the eye. To read a book held at a distance of $25\,cm$, he will need spectacles containing:
A) Chromatic lenses
B) Prismatic lenses
C) Concave lenses
D) Convex lenses

Answer 1

Hint: To solve this question, approach it with the thought about the lens that you need in this case should take the book held at $25\,cm$ as a real object and then deviate or converge the light in such a way that it makes a real image of the book at a distance of $40\,cm$ from the eye since this is the new near point of the long-sighted person.

Complete step by step answer:
We will approach the solution exactly the same way through which we explained in the hint section of the solution to the asked question. We need to consider a lens which takes the actual book in as a real object and produces its real image at a distance of the new near point of the long-sighted person. We can observe that for this to happen, we need a lens which actually converges the incident light rays when they go through it, since the image should be formed on the same side of the lens but even further from where the object actually is, this can only be done when the light rays bend and converge, thus, making the image on the same side as object but even farther away from it.
The lens which is capable of converging the incident light rays is the Convex lens out of the given four options,
Hence, option (D) is the correct answer.

Note: You can even find the focal length of the lens required by applying the lens formula and putting in the value of the object distance $\left( {u = - 25\,cm} \right)$ and image distance $\left( {v = - 40\,cm} \right)$ and thus find the value of the focal length using the formula:
$\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$