Question

In the figure, an air lens of radii of curvature $10 \,cm$ $\left(R_{1}=R_{2}=10\, cm \right)$ is cut in a cylinder of glass $(\mu=1.5)$. The focal length and the nature of the lens is

• A. 15 cm, concave
• B. 15 cm, convex
• C. $\infty$, neither concave nor convex
• D. 0 , concave

Answer 1

Answer: (A)

$\frac{1}{f}=\left({ }_{g} \mu_{a}-1\right)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$
$=\left(\frac{2}{3}-1\right)\left(\frac{2}{10}\right)$
$\Rightarrow f=-15 \,cm$, so behaves as concave lens.