A person who can see things most clearly at a distance of 10 cm requires spectacles to enable him to see clearly things at a distance of 30 cm. The focal length of the spectacles is

Question

A person who can see things most clearly at a distance of 10 cm requires spectacles to enable him to see clearly things at a distance of 30 cm. The focal length of the spectacles is (A) 10 cm (B) 18 cm (C) 30 cm (D) 15 cm

Tardigrade Admin · Accepted Answer

For viewing far objects, concave lenses are used and for concave lens u= wants to see =-30 v= can see =-10 So from (1/f) =(1/v)-(1/u) ⇒ (1/f) =(1/-10)+(1/30) (1/f) =(-3 +1/30) f =-15 cm

Q. A person who can see things most clearly at a distance of $10 c m$ requires spectacles to enable him to see clearly things at a distance of $30 c m$ . The focal length of the spectacles is

10 cm

18 cm

30 cm

15 cm

For viewing far objects, concave lenses are used and for concave lens
$u =$ wants to see $= - 30$
$v =$ can see $= - 10$
So from $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
$\Rightarrow \frac{1}{f} = \frac{1}{- 10} + \frac{1}{30}$
$\frac{1}{f} = \frac{- 3 + 1}{30}$
$f = - 15 c m$

Q. A person who can see things most clearly at a distance of 10cm requires spectacles to enable him to see clearly things at a distance of 30cm. The focal length of the spectacles is