In the figure below,$PQRS$ denotes the path followed by a ray of light as it travels through three media in succession. The absolute refractive indices of the media are $\mu_1, \mu_2$ and $\mu_3$,respectively. (The line segment $RS$ in the figure is parallel to $PQ$). Then,

In finding out the refractive index of a glass slab,the following observations were made through a travelling microscope: $50$ vernier scale divisions $= 49$ $MSD$; $20$ divisions on the main scale in each $cm$. For a mark on paper:
$MSR = 8.45 \ cm, VC = 26$
For the mark on paper seen through the slab:
$MSR = 7.12 \ cm, VC = 41$
For a powder particle on the top surface of the glass slab:
$MSR = 4.05 \ cm, VC = 1$
($MSR =$ Main Scale Reading,$VC =$ Vernier Coincidence)
The refractive index of the glass slab is:

In the figure shown,$\frac{\sin i}{\sin r}$ is equal to:

$A$ monochromatic beam of light passes from a denser medium into a rarer medium. As a result,

When a light ray enters a medium with refractive index $\mu$,it is observed that the angle of refraction is half the angle of incidence. The angle of incidence is .........

If the wavelength of light in vacuum is $\lambda$,the wavelength in a medium of refractive index $n$ will be