On which principle does an optical fiber work?

$A$ point source of light $S$ is placed at the bottom of a vessel containing a liquid of refractive index $\mu = 5/3$. $A$ person is viewing the source from above the surface. There is an opaque disc $D$ of radius $r = 1\, cm$ floating on the surface of the liquid. The centre of the disc lies vertically above the source $S$. The liquid from the vessel is gradually drained out through a tap. The maximum height $h$ of the liquid for which the source cannot be seen at all from above is: (in $, cm$)

If the critical angle for total internal reflection from a medium to vacuum is $30^o$,the velocity of light in the medium is

The phenomenon utilised in an optical fibre is

Critical angle is that angle of incidence in the denser medium for which the angle of refraction in the rarer medium is......$^o$.

$A$ ray of light travelling in a transparent medium of refractive index $\mu$ falls on a surface separating the medium from air at an angle of incidence of $45^o$. For which of the following values of $\mu$ can the ray undergo total internal reflection?