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
Show Answer
Answer:
Correct Answer: 16. (c)
Solution:
- 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.
