What is the unit of refractive index?

What will be the angle of refraction for a light ray incident normally at the surface (in $^{\circ}$)?

Define the following terms in relation to a concave spherical mirror (any five):
$(i)$ Pole
$(ii)$ Centre of curvature
$(iii)$ Radius of curvature
$(iv)$ Principal axis
$(v)$ Principal focus
$(vi)$ Aperture
$(vii)$ Focal length

$A$ beam of light is incident through the holes on side $A$ and emerges out of the holes on the other face of the box as shown in the Figure. Which of the following could be inside the box?

If an object is placed between two plane mirrors inclined at an angle of $45^{\circ}$,then the number of images formed will be $9$. State whether this statement is True or False.

What is the velocity of light in a glass slab of refractive index $1.5$?