Question

Two transparent media of refractive indices ${\mu }_1$ and ${\mu }_3$ have a solid lens shaped transparent media of refractive index ${\mu }_2$ between them as shown in figure.


The correct relation between refractive indices is:

• A. ${\mu }_1>{\mu }_2>{\mu }_3$
• B. ${\mu }_3>{\mu }_2>{\mu }_1$
• C. ${\mu }_2>{\mu }_3>{\mu }_1$
• D. ${\mu }_3>{\mu }_2={\mu }_1$

Answer 1

The correct option is B ${\mu }_3>{\mu }_2>{\mu }_1$

When incident ray is striking the spherical surface of lens, it is getting refracted away from normal.


That means incident ray is going from denser to rarer medium.
$\therefore {\mu }_3>{\mu }_2$

Again, at the plane surface of the lens, the refracted ray is bent away from the normal $\left(N^{\prime }\right)$ to the surface.
$\therefore {\mu }_2>{\mu }_1$

Thus, ${\mu }_3>{\mu }_2>{\mu }_1$

Why this question? Bottom line: Instead of visualizing the lens as a complex system, let us apply the laws of refraction at both the surfaces (spherical and plane) of lens. Tip: Light ray will bend towards normal if it travelling from a rarer to denser medium.