Question

A spherical air bubble in water will act as a :

• A. Convex lens
• B. Concave lens
• C. Plane glass plate
• D. Plano-concave lens

Answer 1

The correct option is B Concave lens

A spherical air bubble has air inside it so, its $\mu$ is less than water. So by lens maker formula

$\frac{1}{f}=(\frac{{\mu }_2}{{\mu }_1}-1)(\frac{1}{R_1}-\frac{1}{R_2})$

Here ${\mu }_2$ is R.I. of material of lens i.e air and ${\mu }_1$ is R.I of surrounding medium.

So, ${\mu }_2<{\mu }_1,$ $(\because \mu ofair<\mu ofwater)$

So, $(\frac{{\mu }_2}{{\mu }_1}-1)$ is negative and $(\frac{1}{R_1}-\frac{1}{R_2})$ is positive

So, $\frac{1}{f}$ is negative

$\Rightarrow$ $f$ is negative

Hence air bubble will act as a concave lens, because concave lens have negative focal lengths.