Question

A convex lens, of focal length $30\, cm$, a concave lens of focal length $120 \,cm$, and a plane mirror are arranged as shown. For an object kept at a distance of $60\, cm$ from the convex lens, the final image, formed by the combination, is a real image, at a distance of :

• A. $60 \,cm$ from the convex lens
• B. $60 \,cm$ from the concave lens
• C. $70 \,cm$ from the convex lens
• D. $70 \,cm$ from the concave lens

Answer 1

Answer: (A)

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{30}=\frac{1}{v}+\frac{1}{60}$
$\frac{1}{60}=\frac{1}{v}$
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{-120}=\frac{1}{v}-\frac{1}{40}$
$\frac{1}{v}=\frac{1}{60}$
Virtual object 10 cm behind plane mirror.
Hence real image 10 cm in from of mirror.
hence 60 cm from convex lens.