$A$ fish looking from within water sees the outside world through a circular horizon. If the fish is $\sqrt{7} \ cm$ below the surface of water,what will be the radius of the circular horizon in $cm$?

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.

$A$ monochromatic beam of light is incident at the interface of two materials of refractive index $n_{1}$ and $n_{2}$ as shown. If $n_{1} > n_{2}$ and $\theta_{C}$ is the critical angle,then which of the following statements is $NOT$ true?

In the given figure,at the water-air interface,a light ray is incident at the critical angle. Find the value of $\mu_g$.

$A$ planar structure of length $L$ and width $W$ is made of two different optical media of refractive indices $n_1=1.5$ and $n_2=1.44$ as shown in the figure. If $L \gg W$,a ray entering from end $AB$ will emerge from end $CD$ only if the total internal reflection condition is met inside the structure. For $L = 9.6 \ m$,if the incident angle $\theta$ is varied,the maximum time taken by a ray to exit the plane $CD$ is $t \times 10^{-9} \ s$,where $t$ is. . . . . . . [Speed of light $c = 3 \times 10^8 \ m/s$]

Explain internal reflection and total internal reflection.