An air bubble in a glass slab $(\mu = 1.5)$ is $5 \, cm$ deep when viewed from one face and $2 \, cm$ deep when viewed from the opposite face. The thickness of the slab is......$cm$

An underwater swimmer is at a depth of $12 \, m$ below the surface of water. $A$ bird is at a height of $18 \, m$ from the surface of water,directly above his eyes. For the swimmer,the bird appears to be at a distance from the surface of water equal to ....... $m$ (Refractive index of water is $4/3$).

$A$ ray of light travelling through a rarer medium is incident at a very small angle '$i$' on a glass slab,and after refraction,its velocity is reduced by $25 \%$. The angle of deviation is:

$A$ ray of light is incident normally on a glass slab of thickness $5 \ cm$ and refractive index $1.6$. The time taken by a ray to travel from the source of light to the surface of the slab is the same as the time taken to travel through the glass slab. The distance of the source from the surface is: (in $cm$)

$A$ fish at a depth of $12 \, cm$ in water is viewed by an observer on the bank of a lake. To what height the image of the fish is raised? $....... \, cm$ (Refractive index of lake water $= 4/3$)

$A$ 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