Question

Two concave lenses $L_1$ and $L_2$ are kept in contact with each other. If the space between the two lenses is filled with material of refractive index $\mu =1$, the magnitude of focal length of the combination

• A. Becomes undefined
• B. Remains unchanged
• C. Increases
• D. Decreases

Answer 1

The correct option is D Decreases

Let us assume if two identical lens are combined then effective focal length of the lens become half.
$\frac{1}{f_{net}}={\frac{1}{f}}_{\frac{1}{f}}=\frac{2}{f}$
$f_{net}=\frac{f}{2}$
Hence combination of lens decreases the net focal llength.