Consider three converging lenses $L_1, L_2$ and $L_3$ having identical geometrical construction. The index of refraction of $L_1$ and $L_2$ are ${\mu }_1$ and ${\mu }_2$, respectively. The upper half of the lens $L_3$ has a refractive index ${\mu }_1$ and the lower half has ${\mu }_2$. A point object $O$ imaged at $O_1$ by the lens $L_1$ and at $O_2$ by the lens $L_2$ placed in same position. If, $L_3$ is placed at the same place,

In producing a pure spectrum, the incident light is passed through a narrow slit placed in the focal plane of an achromatic lens because a narrow slit

By properly combining two prisms made of different materials, it is possible to

$n$ number of identical equilateral prisms are kept in contact as shown in figure. If deviation through a single prism is $\delta$. Then ( $n, m$ are integers)

Two plane mirrors $M_1$ and $M_2$ are inclined to each other at an angle $\theta$.  When a ray of light incident on $M_2$, parallel to $M_1$ (as shown) undergoes a total of eight reflections, it emerges parallel to $M_2$. Find the value of $100\theta$ (in radians). (Approximate the answer to the nearest integer.)