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.
What is the closest and farthest distance at which he should keep the lens from the page so that he can read the book, when viewing through the magnifying glass?

Solution

For the closest distance,

Image distance, v = -25 cm
Focal length of thin convex lens, f = 5 cm

Using the formula,
   $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

$∴$ $\frac{1}{u} = \frac{1}{v} - \frac{1}{f}$

   $= \frac{1}{- 25} - \frac{1}{5} = \frac{- 1 - 5}{25} = \frac{- 6}{25}$

$u = - \frac{25}{6} = - 4.2 cm$

i.e., 4.2 cm is the closest distance at which the man can read the book.

For the farthest image,

$Image distance, v = \infty Focal length, f = 5 cm$

Using the formula,
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

   $\frac{1}{u} = \frac{1}{v} - \frac{1}{f}$

$= \frac{1}{\infty} - \frac{1}{5} = - \frac{1}{5}$

   $u = - 5 cm$
which is the object distance.

This is the farthest distance at which the man can read the book.

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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.
What is the closest and farthest distance at which he should keep the lens from the page so that he can read the book, when viewing through the magnifying glass?

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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.What is the closest and farthest distance at which he should keep the lens from the page so that he can read the book, when viewing through the magnifying glass?

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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.
What is the closest and farthest distance at which he should keep the lens from the page so that he can read the book, when viewing through the magnifying glass?