Question

A thin convex lens of refractive index $1.5$ has a focal length of $15cm$ in air . When the lens is placed in liquid of refractive index $4/3$, its focal length will be

• A. $15cm$
• B. $10cm$
• C. $30cm$
• D. $60cm$

Answer 1

The correct option is D $60cm$

As we know,

$\frac{f_l}{f_a}=\frac{{(}_a{\mu }_g-1)}{{}_l{\mu }_g-1}\Rightarrow \frac{f_l}{f_{15}}$$=\frac{(1.5-1)}{(\frac{1.5}{4/3}-1)}$$\Rightarrow d{f}_l=60cm$