NEET Solved Paper 2018 Question 20
Question: An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be [NEET - 2018]
Options:
A) 30 cm towards the mirror
B) 36 cm away from the mirror
C) 30 cm away from the mirror
D) 36 cm towards the mirror
Show Answer
Answer:
Correct Answer: B
Solution:
$ \frac{1}{f}=\frac{1}{v _1}+\frac{1}{u} $ $ -\frac{1}{15}=\frac{1}{v _1}-\frac{1}{40} $
$ \Rightarrow \frac{1}{v _1}=\frac{1}{-15}+\frac{1}{40} $ $ v _1=-24cm $ .
When object is displaced by 20 cm towards mirror.
Now, $ u _2=-20 $ $ \frac{1}{f}=\frac{1}{v _2}+\frac{1}{u _2} $ $ \frac{1}{-15}=\frac{1}{v _2}-\frac{1}{20} $ $ \frac{1}{v _2}=\frac{1}{20}-\frac{1}{15} $ $ v _2=-60cm $
So, image shifts away from mirror by $ =60-24=36cm $ .
