The critical angle for a medium is $60^o$. The refractive index of the medium is:

Assertion $(A)$: Propagation of light through an optical fibre is due to total internal reflection taking place at the core-clad interface.
Reason $(R)$: Refractive index of the material of the core of the optical fibre is greater than that of air.

$A$ beam of light is incident from air on the surface of a liquid. The angle of incidence is $\theta$ and the angle of refraction is $\alpha$. If the critical angle for the liquid when surrounded by air is $\theta_c$,then $\sin \theta_c$ is:

$A$ vertical pencil of rays comes from the bottom of a tank filled with a liquid. When the tank is accelerated horizontally with an acceleration of $7.5\, m/s^2$,the ray is seen to be totally internally reflected by the liquid surface. What is the minimum possible refractive index of the liquid? (Take $g = 10\, m/s^2$)

$A$ light ray falls on a square glass slab as shown in the diagram. The index of refraction of the glass,if total internal reflection is to occur at the vertical face,is equal to

Consider a glass cube slab of dielectric bound by the planes $x=0, x=a$; $y=0, y=b$; $z=0, z=c$; with $b > a > c$. The slab is placed in air and has a refractive index of $n$. The minimum value for $n$,such that all rays entering the dielectric at $y=0$ reach $y=b$ is