A light ray enters a planetary atmosphere vertically. The
atmosphere is 20 km thick. The index of refraction of the
atmosphere increases linearly from a value of 1 at the top of the
atmosphere to a value of 1.5 at the bottom.
a) Determine how long it takes the light ray to travel through
the atmosphere.
b) Compare your answer in part a to the time it would take the
light ray to cover the same distances without the atmosphere.