I usually propose a rather simple, albeit not cheap, experiment for this.

1. Find a sufficiently large "shallow" body of water. 2. Build a number a number of masts/poles in a straight line, all with an identical height over the water.

Now when standing at the shore and looking at the masts I predicts one of two things.

a. you can see all masts in their entirety with a telescope, or b. you can see less and less of the masts the further away they are.

There are actual places with power lines that satisfy this requirement. For b. you can use high school math to reason about the curvature needed to explain the results. There is no such sound logic (to my knowledge) explaining this for a flat surface.