Optics Question 94
Question: The size of the image of an object, which is at infinity, as formed by a convex lens of focal length 30cm is 2 cm. If a concave lens of focal length 20 cm is placed between the convex lens and the image at a distance of 26 cm from the convex lens, calculate the new size of the image
[IIT-JEE (Screening) 2003]
Options:
A) 1.25 cm
B) 2.5 cm
C) 1.05 cm
D) 2 cm
Show Answer
Answer:
Correct Answer: B
Solution:
Convex lens will form image $ I _{1} $ at it’s focus which acts like a virtual object for concave lens.
Hence for concave lens $ u=+4cm $ , $ f=20 $ $ cm. $
So by lens formula $ \frac{1}{-20}=\frac{1}{v}-\frac{1}{4}\Rightarrow v=5cm $ i.e. distance of final image $ (I _{2}) $ from concave lens $ v=5cm $ by using $ \frac{v}{u}=\frac{I}{O}\Rightarrow \frac{5}{4}=\frac{I}{2}\Rightarrow (I _{2})=2.5cm $
