I know how they will respond. This is a pretty famous picture in their circles. These two are the most often mentioned rebuttals:
- There are videos showing the same transmission lines with no curvature:
- There are videos showing that the curvature can be replicated in Flat Earth by throwing in atmospheric refractiong. See for example:YouTube
The key issue with both claims is atmospheric refraction. In 1), during some strong weather conditions the atmospheric refraction can be strong enough to make the lines appear straight. Further, the strong weather conditions in 1) also made the atmosphere hazy that the sight distance is greatly reduced. In 2), the atmospheric refraction that they throw in is completely made up and does not match the conditions when/where the photograph was taken.
If one gives these answers, one must be ready to explain to them atmospheric refraction from a common ground (i.e. physics that they also agree with), and why it bends light the way it does (also why it bends oppositely during mirages). This is an example of needing to be better equipped than knowing high school physics to debate Flat Earthers. Most people who try to debate Flat Earthers only know high school level physics (often less physics than the Flat Earthers), so they cannot properly answer these objections, which entrench the Flat Earthers even more in their position.