Question

The magnification of an object placed in front of a convex lens of focal length 20cm is +2. To obtain a magnification of -2, the object has to be moved by a distance equals to

• A. 10cm
• B. 30cm
• C. 20cm
• D. 40cm

Answer 1

The correct option is C 20cm

$m=\frac{v}{u}=2u=-x,v=-2x,f=+20\text{Using lens formula,}\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\Rightarrow \frac{1}{-2x}+\frac{1}{x}=\frac{1}{20}\Rightarrow x=10cm\text{To have magnification, -2}u=-y,v=2yandf=20cm\Rightarrow \frac{1}{2y}+\frac{1}{y}=\frac{1}{20}\Rightarrow y=30cm\therefore y-x=30-10=20cm$