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Question
A man with normal near point 25 cm reads a book with small print using a magnifying glass, a thin convex lens of focal length 5 cm. The closest and farthest distances from the magnifying glass at which he can read the book when viewing through the magnifying glass, respectively are -
A man with normal near point 25 cm reads a book with small print using a magnifying glass, a thin convex lens of focal length 5 cm. The closest and farthest distances from the magnifying glass at which he can read the book when viewing through the magnifying glass, respectively are -
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Solution
The correct option is A 4.17 cm,5 cm
The magnifying glass must form the image of the book between 25 cm (least distance of distinct vision) and ∞
For image formed at 25 cm from the eye,
v=−25 cm (since magnifying glass is held close to the eye and it forms virtual image)
and given f=5 cm
Using lens formula, 1u=1v−1f=1−25−15
⇒u=−256=−4.17 cm
For image formed at ∞, object should be placed at focus i.e at 5 cm from the lens.
Thus, the book should be placed between 4.17 cm and 5 cm from the magnifying glass.
The magnifying glass must form the image of the book between 25 cm (least distance of distinct vision) and ∞
For image formed at 25 cm from the eye,
v=−25 cm (since magnifying glass is held close to the eye and it forms virtual image)
and given f=5 cm
Using lens formula, 1u=1v−1f=1−25−15
⇒u=−256=−4.17 cm
For image formed at ∞, object should be placed at focus i.e at 5 cm from the lens.
Thus, the book should be placed between 4.17 cm and 5 cm from the magnifying glass.
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