When a light ray travels from an optically denser medium to an optically rarer medium, what is the angle of incidence called at which the angle of refraction becomes $90^{\circ}$?

How many times is the velocity of light in a vacuum compared to the velocity of light in water?

The path of a ray of light coming from air passing through a rectangular glass slab traced by four students are shown as $A, B, C$ and $D$ in the figure. Which one of them is correct?

$A$ pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent,if instead of water we use liquids like kerosene or turpentine? Support your answer with reason.

What is the velocity of light in a glass slab of refractive index $1.5$?

State the laws of refraction.