I'm researching the credibility of FET for a school project and came up with the question, "If the Earth is flat, why is it clearly visible that the sun sets/rises beyond the horizon (visually) instead of simply moving away from us on the same altitudinal plane?"

After further research, I found that people attempt to explain this using the "Law of Perspective."

Can someone please either answer my question or, if that is not possible without using the Law of Perspective, explain to me exactly what that is and give me proof, or at least evidence that this "law" applies to our perception of the sun moving across a flat Earth?