Optics Question 631
Question: An object is placed first at infinity and then at 20 cm from the object side focal plane of the convex lens. The two images thus formed are 5 cm apart. The focal length of the lens is
[SCRA 1994]
Options:
A) 5 cm
B) 10 cm
C) 15 cm
D) 20 cm
Show Answer
Answer:
Correct Answer: B
Solution:
For first case : $ \frac{1}{f}=\frac{1}{v}-\frac{1}{\infty }\Rightarrow f=v $
For second case $ \frac{1}{f}=\frac{1}{(f+5)}-\frac{1}{-(f+20)}\Rightarrow f=10cm $
Alternative sol.: $ f^{2}=x _{1}x _{2} $
Therefore $ f=10cm. $
