The distance between the object kept at $2 \, m$ from a plane mirror and its image is $.....$ (in $, m$)

$A$ ray of light enters from a medium with refractive index $n_1$ into a medium with refractive index $n_2$. If the angle of incidence is $\theta_1$ and the angle of refraction is $\theta_2$,then $\sin \theta_1 / \sin \theta_2 = \dots$

Discuss the position and nature of the image formed by a concave mirror when the object is moved from infinity towards the pole of the mirror.

What are the units of refractive index?

State the sign convention used to measure the length of the image and the object.

$A$ convex lens has a focal length of $25\, cm$. Calculate the distance of the object from the lens if the image is to be formed on the opposite side of the lens at a distance of $75\, cm$ from the lens. What will be the nature of the image?