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

$A$ fish is a little below the surface of a lake. If the critical angle is $49^\circ$,then the fish could see things above the water surface within an angular range of $\theta^\circ$ where

Assertion $(A)$: Optical fibres are widely used in communication networks. Reason $(R)$: Optical fibres are small in size, light in weight, flexible, and there is no scope for interference in them.

Assertion : In optical fibre,the diameter of the core is kept small.
Reason : This smaller diameter of the core ensures that the fibre should have incident angle more than the critical angle required for total internal reflection.

In the figure shown,for an angle of incidence $45^{\circ}$ at the top surface,what is the minimum refractive index $\mu$ needed for total internal reflection at the vertical face?

$A$ ray of light travels from a denser medium $(\mu)$ to air. The angle of incidence is denoted by $i$ and the angle of deviation by $D$. Let $C = \sin^{-1}(1/\mu)$. Which of the following graphs is correct?