$A$ 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 $cm$?

In an optical fibre, the core and cladding are made with materials of refractive indices $1.5$ and $1.414$ respectively. To observe total internal reflection, what will be the range of the incident angle with the axis of the optical fibre?

It is found that electromagnetic signals sent inside a glass sphere from $A$ towards $B$ reach point $C$. The speed of electromagnetic signals in glass cannot be:

If the critical angle for a material to air is $30^o$,the refractive index of the material will be

$A$ ray of light is incident at an angle $i$ from a denser to a rarer medium. The reflected and the refracted rays are mutually perpendicular. The angle of reflection is $r$ and the angle of refraction is $r'$. Then the critical angle $C$ will be:

$A$ ray of light from a denser medium strikes a rarer medium. The angle of reflection is $r$ and that of refraction is $r'$. The reflected and refracted rays make an angle of $90^{\circ}$ with each other. The critical angle will be