Question

A convex lens of focal length 12 cm is made of glass of $\mu =\frac{5}{4}?$ What will be its focal length when immersed in liquid of $\mu =\frac{5}{4}?$

• A. 6 cm
• B. 12 cm
• C. 24 cm
• D. 30 cm

Answer 1

Answer: (D)

30 cm

As we know,

\$\frac{f_l}{f_a}=\frac{{(}_a{\mu }_g-1)}{{(}_l{\mu }_g-1)}=\frac{(\frac{3}{2}-1)}{(\frac{3/2}{5/4}-1)}=\frac{5}{3}$
$\therefore f_1=f_a\left(\frac{5}{2}\right)=\frac{12\times 5}{2}=30cm$