Optics Question 813
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:
Options:
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
Show Answer
Answer:
Correct Answer: A
Solution:
[a] For convex lens, $ \frac{1}{30}=\frac{1}{v}+\frac{1}{60}\Rightarrow \frac{1}{60}=\frac{1}{v} $
Similarly for concave lens $ \frac{-1}{-120}=\frac{1}{v}-\frac{1}{40}\Rightarrow \frac{1}{v}=\frac{1}{60} $
Virtual object 10 cm behind plane mirror.
Hence real image 10 cm in front of mirror or, 60 cm from convex lens.
