Question

To get an image of magnification –1 on a screen using a lens of focal length 20 cm, the object distance must be : 

(a) Less than 20 cm 

(b) 30 cm 

(c) 40 cm 

(d) 80 cm

Answer 1

Correct option is (c) 40 cm

\(m=\frac{f}{f+u}\)

\(-1=\frac{20}{20+u}\)

\(-20-u=20\)

\(u=-40cm\)