In an optical fibre,the refractive index of the inner part (core) is $1.68$ and the refractive index of the outer part (cladding) is $1.44$. The numerical aperture of the fibre is ........

$A$ diver in a swimming pool wants to signal his distress to a person lying on the edge of the pool by flashing his waterproof flashlight. In which direction should he direct the beam?

$A$ 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

Explain the phenomenon of total internal reflection in a right-angled prism.

Material $A$ has a critical angle ${i_A},$ and material $B$ has a critical angle ${i_B}$ $({i_B} > {i_A})$. Then which of the following is true?
$(i)$ Light can be totally internally reflected when it passes from $B$ to $A$.
$(ii)$ Light can be totally internally reflected when it passes from $A$ to $B$.
$(iii)$ The critical angle for total internal reflection is ${i_B} - {i_A}$.
$(iv)$ The critical angle between $A$ and $B$ is ${\sin ^{ - 1}}\left( {\frac{{\sin {i_A}}}{{\sin {i_B}}}} \right)$.

What is the maximum acceptance angle at the entrance surface of an optical fiber,if $n_1$ and $n_2$ are the refractive indices of the core and cladding,respectively?