If ${ }_{i} \mu_{j}$ represents the refractive index when a ray goes from medium $i$ to medium $j$,then the product ${ }_2 \mu_1 \times { }_3 \mu_2 \times { }_4 \mu_3$ is equal to:

$A$ monochromatic light wave with wavelength $\lambda_1$ and frequency $v_1$ in air enters another medium. If the angle of incidence and angle of refraction at the interface are $45^{\circ}$ and $30^{\circ}$ respectively,then the wavelength $\lambda_2$ and frequency $v_2$ of the refracted wave are :

The distance travelled by light in glass (refractive index $= 1.5$) in a nanosecond will be.......$cm$.

$A$ cubical vessel has opaque walls. An observer is located such that she can see only the wall $CD$,but not the bottom. To what height (in $cm$) should water be poured so that she can see an object placed at the bottom at a distance of $10 \, cm$ from the corner $C$? The refractive index of water is $\mu = 1.33 \approx 4/3$.

Give the meaning of $n_{12}$.

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