Light travels through a glass plate of thickness $t$ and having refractive index $n$. If $c$ is the velocity of light in vacuum,the time taken by the light to travel this thickness of glass is

The refractive index of air with respect to vacuum is . . . . . . .

The difference of speed of light in the two media $A$ and $B$ $(v_{A}-v_{B})$ is $2.6 \times 10^{7} \, m/s$. If the refractive index of medium $B$ is $1.47$,then the ratio of refractive index of medium $B$ to medium $A$ is: (Given: speed of light in vacuum $c = 3 \times 10^{8} \, m/s$)

$A$ ray of light is incident on a medium of refractive index $\mu$ at an angle of incidence $i$. On refraction in the medium,$\delta$ is the angle of deviation. Then:

If $n_{21} > 1$,then write the relation between the angle of incidence and the angle of refraction.

Give the meaning of $n_{12}$.