Question

A magnifying glass is to be used at the fixed object distance of 1 inch. If it is to produce an erect image 5 items magnified, its focal length should he :

(a) 0.2"

(b) 0.8"

(c) 1.2"

(d)5"

Answer 1

Correct answer is (d)5"

Answer 2

M=v/u.  V=5inch (given u=1 inch)

Using sign convention u=-1inch. V=-5inch

 1/f=1/v-1/u.   

1/-5-1/-1

F=1.25"