First slide

The lens

Question

Two convex lenses placed in contact form the image of a distant object at P. If the lens B is moved to the right, the image will

Moderate

A

move to the left
B

move to the right
C

remain at P
D

move either to the left or right, depending upon focal

Solution

$\begin{array}{l} I n i t i a l l y, w i t h o u t g a p : \\ \frac{1}{F_{i}} = \frac{1}{f_{1}} + \frac{1}{f_{2}} \\ F i n a l l y, w i t h g a p : \\ \frac{1}{F_{f}} = \frac{1}{f_{1}} + \frac{1}{f_{2}} - \frac{d}{f_{1} f_{2}} \\ C o m p a r i n g, F_{f} > F_{i} \end{array}$
Power of the system decreases due to separation between the lenses. So the focal length increases.

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