Question

A man suffering from myopia can read a book placed at 10 cm distance. For reading the book at a distance of 60 cm with relaxed vision, focal length of the lens required wilbe

• A. 45 cm
• B. -12 cm
• C. -20 cm
• D. 30 cm

Answer 1

The correct option is B -12 cm

We know that: $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

Since, man suffering from myopia can see at distance of 10 cm clearly and he wants to read book at 60 cm, so he will use a lens that can make image of 60 cm at 10 cm, therefore,

$u=-60cmandv=-10cm$


Now applying lens formula we will get f,

$\Rightarrow \frac{1}{f}=\frac{1}{-10}-\frac{1}{-60}=-\frac{5}{60}$

$\Rightarrow f=-12cm$

Final answer : (C)