Q. 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
A) 5$cm$
B) 10 $cm$
C) 15 $cm$
D) 20 $cm$
Explanation
(b) For first case : $\frac{1}{f} = \frac{1}{v} - \frac{1}{\infty} \Rightarrow f = v$
For second case $\frac{1}{f} = \frac{1}{(f + 5)} - \frac{1}{- (f + 20)} \Rightarrow f = 10\ cm$
Alternative sol. $- f^{2} = x_{1}x_{2} \Rightarrow f = 10\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.