For which type of mirror will the image always be erect,regardless of the distance at which the object is placed?

$(a)$ Define absolute refractive index.
$(b)$ The path of a light ray from three different media $A, B$ and $C$ for a given angle of incidence is shown below. Study the diagrams and answer the following questions.
$(i)$ Which of the three media $A, B$ or $C$ has maximum optical density?
$(ii)$ Through which of the three media,will the speed of light be maximum?
$(iii)$ Will the light travelling from $A$ to $B$ bend towards or away from the normal?
$(iv)$ Will the refractive index of $B$ relative to $C$ be more than unity or less than unity?

An object is placed at the following distances from a concave mirror of focal length $15 \, cm$.
$(a)$ $10 \, cm$ $(b)$ $20 \, cm$ $(c)$ $30 \, cm$ $(d)$ $40 \, cm$
Which position of the object will produce:
$(1)$ Virtual image
$(2)$ $A$ diminished real image
$(3)$ An enlarged real image
$(4)$ An image of same size

Which of the following ray diagrams is correct for the ray of light incident on a lens shown in the figure?

In which of the following,the image of an object placed at infinity will be highly diminished and point-sized?

Which type of mirror can produce a virtual image?