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 will be

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 will be (A) 45 cm (B) -20 cm (C) -12 cm (D) 30 cm

Tardigrade Admin · Accepted Answer

For lens u= wants to see =-60 cm v= can see =-10 cm ∴ (1/f)=(1/v)-(1/u) ⇒ (1/f)=(1/-10)-(1/(-60)) ⇒ f=-12 cm

Q. A man suffering from myopia can read a book placed at $10 c m$ distance. For reading the book at a distance of $60 c m$ with relaxed vision, focal length of the lens required will be

45 cm

-20 cm

-12 cm

30 cm

For lens $u =$ wants to $see = - 60 c m$
$v =$ can see $= - 10 c m$
$∴ \frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
$\Rightarrow \frac{1}{f} = \frac{1}{- 10} - \frac{1}{( - 60 )}$
$\Rightarrow f = - 12 c m$

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