$A$ point source of light is placed $4 \; m$ below the surface of water of refractive index $5/3$. The minimum diameter of a disc which should be placed over the source on the surface of water to cut-off all light coming out of water is ... $m$.

$A$ beam containing red,green,and blue light is incident on a prism. Their refractive indices are $1.39, 1.44$,and $1.47$ respectively. Then,the prism:

$A$ ray of laser of a wavelength $630 \, nm$ is incident at an angle of $30^{\circ}$ at the diamond-air interface. It is going from diamond to air. The refractive index of diamond is $2.42$ and that of air is $1$. Choose the correct option.

Three rays of light,namely red $(R)$,green $(G)$ and blue $(B)$ are incident on the face $PQ$ of a right-angled prism $PQR$ as shown in the figure. The refractive indices of the material of the prism for red,green and blue wavelengths are $1.27$,$1.42$ and $1.49$ respectively. The colour of the ray$(s)$ emerging out of the face $PR$ is:

Assertion: Optical fibres are used for telecommunication.
Reason: Optical fibres are based on the phenomenon of total internal reflection.

$A$ 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