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

Step-by-step solution

A concave lens is required by the person so that the lens forms the image of object distant 30 cm , at 10 cm it means aligned u & =-30 ~cm , v=-10 ~cm 1 f & = 1 v - 1 u & = 1 -10 - 1 -30 =- 2 30 or 1 f & = -1 15 So f & =-15 ~cm aligned Minus sign singnifies that lens used is concave