Write two situation when angle of refraction is `90^@`.

What change occurs in the angle of refraction with the increase in the angle of incidence when a ray enters a medium ?

What speciality is observed in the value of ratio of sine of the angle of incidence to the sine of the angle of refraction, sin I / sin r ?

Write two situations which is related to Boyle's law.

Write down situation related to each Newton's third law of motion