Question

A convex lens made of glass of $\mu =1.5$ has a focal length $f$ in air. When it is completely immersed in a liquid of $\mu$ $=2$ its focal length and nature will be:

• A. $2f$, converging
• B. $3f$, converging
• C. $2f$, diverging
• D. $3f$, diverging

Answer 1

The correct option is C $2f$, diverging

Let $k=$$\frac{1}{R_1}-\frac{1}{R_2}$

So, $\frac{1}{f}=(\mu -1)k$

or, $\frac{1}{f}=(\frac{3}{2}-1)k$

or, $k=$ $\frac{2}{f}$
Also, in another medium,
$\frac{1}{f^{\prime }}=(\frac{1.5}{2}-1)k$
Thus, putting $k=\frac{2}{f}$, we get: $f^{\prime }=-2f$