The critical angle for diamond (refractive index $= 2$) is........$^o$

For a light ray to undergo total internal reflection ($i =$ angle of incidence,$i_c =$ critical angle):

Light travels in two media $A$ and $B$ with speeds $1.8 \times 10^8 \ m/s$ and $2.4 \times 10^8 \ m/s$ respectively. Then the critical angle between them is

$A$ small bulb is placed at the bottom of a tank containing water to a depth of $\sqrt{7} \; m$. The refractive index of water is $\frac{4}{3}$. The area of the surface of water through which light from the bulb can emerge out is $x \,\pi \; m^{2}$. The value of $x$ is ...............

$A$ light ray falls on a glass surface of refractive index $\sqrt{3}$,at an angle of $60^{\circ}$. The angle between the refracted and reflected rays would be ....... $^o$.

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.