Question

Figure (a) shows two plano-convex lenses in contact as shown. The combination has focal length $24cm$. Figure (b) shows the same with a liquid introduced between them. If refractive index of glass of the lenses is $1.50$ and that of the liquid is $1.60$, the focal length of system in Fig. (b) will be

• A. $-120cm$
• B. $120cm$
• C. $-24cm$
• D. $24cm$

Answer 1

Answer: (A)

$-120cm$

as initially $f=24$ => $R=24$

$\frac{1}{f}=\frac{1}{24}+\frac{2\times 0.6}{-24}$

$f=-120$

option $A$ is correct