Optics Question 930
Question: A point object is placed at a distance of $ 10cm $ and its real image is formed at a distance of $ 20cm $ from a concave mirror. If the object is moved by $ 0.1cm $ towards the mirror, the image will shift by about
[MP PMT 2000]
Options:
A) $ 0.4cm $ away from the mirror
B) $ 0.4cm $ towards the mirror
C) $ 0.8cm $ away from the mirror
D) $ 0.8cm $ towards the mirror
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Answer:
Correct Answer: A
Solution:
Mirror formula $ \frac{1}{f}=\frac{1}{v}+\frac{1}{u}\Rightarrow \frac{1}{f}=\frac{1}{-20}+\frac{1}{(-10)}\Rightarrow f=\frac{20}{3}cm. $
If object moves towards the mirror by $ 0.1cm $
then. $ u=(10-0.1)=9.9cm. $
Hence again from mirror formula $ \frac{1}{-20/3}=\frac{1}{{{v}’}}+\frac{1}{-9.9}\Rightarrow {v}’=20.4cm $ i.e. image shifts away from the mirror by $ 0.4cm. $
