$A$ transparent glass cube of length $24 \,cm$ has a small air bubble trapped inside. When seen normally through one surface from air outside, its apparent distance is $10 \,cm$ from the surface. When seen normally from the opposite surface, its apparent distance is $6 \,cm$. The distance of the air bubble from the first surface is (in $\,cm$)

Time taken by the sunlight to pass through a slab of $4 \ cm$ and refractive index $1.5$ is . . . . . . $s$.

$A$ glass slab of thickness $4 \,cm$ contains the same number of waves as in $x \,cm$ of water column when both are traversed by the same monochromatic light. If the refractive indices of glass and water for that light are $\frac{5}{3}$ and $\frac{4}{3}$ respectively,the value of $x$ will be

$A$ plane glass slab is kept over a coloured word '$VIBGYOR$',where the colour of each letter corresponds to the colours in white light. The letter which appears least raised is

$A$ glass slab has a thickness $t$ and a refractive index $\mu$. Calculate the time taken by light to travel through this thickness.

The back surface of a glass slab (refractive index $n$ and thickness $t$) is polished to act as a mirror,as shown in the figure. $A$ laser beam is incident on it,which is partially reflected and partially refracted at the air-glass interface. The refracted part is fully reflected at the mirror surface. $A$ pattern of discrete spots of light is observed on a screen placed perpendicular to the glass surface. The spacing between the spots on the screen is: