Question

As we come near down to the surface of the earth from a distant star, how does the refractive index vary?

Answer 1

If you assume Earth to be a perfect sphere of constant density, then the gravitational force would start diminishing immediately in a linear fashion, until it reached zero in the centre. However, there is one very glaring problem with the above assumption: Earth is not a uniform ball of material. Indeed, Earth’s core is more than three times denser than the surface material. That means that for a significant portion of Earth’s radius, the effect of less dense material being above you does not adequately counteract the fact that you are getting closer to the denser stuff. As such, instead of diminishing linearly, the gravitational force actually stays constant for most of the trip downwards, rises slightly at core-mantle boundary, and only then starts dropping off as expected[1].

TLDR: Earth’s density is not uniform, with most material concentrated at the  core

Answer 2

Answer:

If you assume Earth to be a perfect sphere of constant density, then the gravitational force would start diminishing immediately in a linear fashion, until it reached zero in the centre. However, there is one very glaring problem with the above assumption: Earth is not a uniform ball of material. Indeed, Earth’s core is more than three times denser than the surface material. That means that for a significant portion of Earth’s radius, the effect of less dense material being above you does not adequately counteract the fact that you are getting closer to the denser stuff. As such, instead of diminishing linearly, the gravitational force actually stays constant for most of the trip downwards, rises slightly at core-mantle boundary, and only then starts dropping o

Explanation: