A light ray enters the atmosphere of a planet where it descends vertically to the surface a

Question:

A light ray enters the atmosphere of a planet where it descends vertically to the surface a distance h below. The index of refraction where the light enters the atmosphere is 1.000, and it increases linearly to the surface where it has the value n.
(a) How long does it take the ray to traverse this path?
(b) Compare this to the time interval required in the absence of an atmosphere.