A person who can see things most clearly at a distance of `10 cm`. Requires spectacles to enable to him to see clearly things at a distance of `30 cm.` What should be the focal length of the spectacles ?

Explanation

Since, the personis not able to see distant object beyond 10 cm. he should use concave lens in his spectacles.
Concave lens will form the image of object distant 30 cm, at 10 cm it means u=-30 cm,v=-10 cm.
Using lens formula,
`1/f=1/v-1/u=1/(-10)-1/(-30)=2/30or1/f=1/15`
So, f=-15 cm
Minus sign signifies that lens used in concave.