Three lenses in contact have a combined focal length of 12 cm. When the third lens is removed the combined focal length is 60/7 cm. The third lens is

see full answer

Take the SCORE scholarship exam from home and compete for scholarships worth ₹1 crore!*

An Intiative by Sri Chaitanya

A converging lens of focal length 30 cm.

A converging lens of focal length 60 cm.

A diverging lens of focal length 30 cm.

A diverging lens of focal length 60 cm.

answer is C.

(Unlock A.I Detailed Solution for FREE)

Ready to Test Your Skills?

Check your Performance Today with our Free Mock Test used by Toppers!

Take Free Test

Detailed Solution

$\begin{aligned} f = 12 \\ \frac{1}{f} = \frac{1}{f_{1}} + \frac{1}{f_{2}} + \frac{1}{f_{3}} \end{aligned}$

When third lens is removed

$\frac{1}{f^{'}} = \frac{1}{f_{1}} + \frac{1}{f_{2}}$

$\begin{aligned} f^{'} = \frac{60}{7} \\ \frac{1}{12} = \frac{1}{\frac{60}{7}} + \frac{1}{f_{3}} \\ \Rightarrow f_{3} = - 30 \end{aligned}$

Watch 3-min video & get full concept clarity