Optics 3 Question 16

16. A point object is placed at a distance of $20 cm$ from a thin plano-convex lens of focal length $15 cm$. The plane surface of the lens is now silvered. The image created by the system is at

(2006, 3M)

(a) $60 cm$ to the left of the system

(b) $60 cm$ to the right of the system

(c) $12 cm$ to the left of the system

(d) $12 cm$ to the right of the system

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Answer:

Correct Answer: 16. (c)

Solution:

  1. Refraction from lens : $\frac{1}{v _1}-\frac{1}{-20}=\frac{1}{15}$

$\therefore \quad v=60 cm \quad+$ ve direction

i.e. first image is formed at $60 cm$ to the right of lens system.

Reflection from mirror

After reflection from the mirror, the second image will be formed at a distance of $60 cm$ to the left of lens system.

Refraction from lens

or

$$ \frac{1}{v _3}-\frac{1}{60}=\frac{1}{15} \quad \leftarrow \quad+\text { ve direction } $$

Therefore, the final image is formed at $12 cm$ to the left of the lens system.



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