In an optical fibre, is the refractive index of the core greater than that of the cladding, or smaller?

$A$ 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 $g = 10\, m/s^2$)

What is total internal reflection?

$A$ glass prism of refractive index $1.5$ is immersed in water (refractive index $\frac{4}{3}$) as shown in the figure. $A$ light beam incident normally on the face $AB$ is totally internally reflected at the face $AC$ to reach the face $BC$,if:

$A$ fish inside water sees the outside world through a circular horizon. If the refractive index of water is $4/3$ and the fish is $12 \, cm$ below the water surface,then the radius of this circle is $= ..... cm$.

Why is mirage formation seen in deserts during the summer?