A far sighted person has a near point of `60"cm"` . What power lens should be used for eye glasses such that the person can be read this book at a distance of `25` cm.

Options

A. ` + 1.67` D
B. ` + 2 ` D
C. ` + 2.33` D
D. ` - 1.67 ` D

Answer

C

Explanation

It is given that his far sight is good beyond 60 cm. His near sight is bad. If he desires to read book at ` u = - 25 ` cm , then for good vision image distance should be at a distance of 60 cm or more. For minimum power lens, ` v =- 60 cm , u = -25 cm ` .
Therefore the focal length of the lens used should be such that
` ( 1 ) /( f ) = ( 1 ) /( v ) - ( 1 ) /( u ) = - ( 1 ) /( 60 ) + ( 1 ) /( 25 ) rArr f = (300)/( 7 ) cm `
` therefore ` Power P = ` ( 1 ) /( "f ( in metre )") = ( 1 ) /(( 3//7 ) ) = 2.33 D `