Suppose we build a highway through a terrain that is such that curves must be construct in some areas.
Now suppose that our highway is a ideal short cut from A to B and we use it regularly for transporting necessary cargo back and forth.
After a while a swamp develops near one of the curves and a frog comes out of it onto the road.
Our vehicles will not be hindered by one frog, but what if there is a extreme frog population growth and thousands of them pile up on the road at the same time, that would constitute a danger because it would make the road slippery and being close to a curve the danger of accidents or loss of cargo and life would be multiplied.
At least the traffic would need to slow down and the delivery of important goods would be delayed.

Would you not be curious as to why the frogs are on the road and what could be done about it?

I’m sure that few heavy nuclei from uranium atoms in the upper reaches of the atmosphere would not constitute a danger for space ships of either earth type or ET type, but at great densities such molecules could hinder high velocity motion of craft in areas where such atomic particles would exist.
If nothing else, it would create more friction and heat on the earth type craft.

What would make it impossible for “atmospheric refraction” to operate at higher density atmosphere?

Thank you for clarifying my understanding.