Question

A concave lens made of a material of refractive index $\frac{3}{2}$is placed in a transparent liquid of refractive index $1.8$. A parallel beam of light rays after passing through the lens -

• A. diverges
• B. converges
• C. undergoes not refraction
• D. emerges out as a parallel beam itself

Answer 1

The correct option is B

converges

Step 1: Recall Lens Maker Formula:

1. We know the lens maker formula $\frac{1}{f}=-\left(\frac{{\mu }_1}{{\mu }_2}-1\right)\left(\frac{1}{R_1}+\frac{1}{R_2}\right)$ , where ${\mu }_1$ is the refractive index of the surrounding medium and ${\mu }_2$ is the refractive index of the material of the lens.

Step 1: Apply the given data in the lens maker formula

1. Here, $\frac{{\mu }_1}{{\mu }_2}=\frac{1.5}{1.8}<1$ so, $\frac{{\mu }_1}{{\mu }_2}-1<0$ from the lens maker formula we can see that $\frac{1}{f}>0$.
2. So, the focal length of the lens is positive.
3. Therefore, according to the sign convention, the lens will be converging.

So, the correct option is (B) converges.