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.



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