Radio waves are reflected by the atmosphere (hence why we don’t need to set up several towers to transmit radio waves across very long distances).

Light waves are also reflected in a similar way. When we look across, the light diminishes as it travels, and by the time it reaches the atmosphere, it is diminished enough to be reflected.

The light will then hit another corner of atmosphere and so on and so forth, curving every time. This gives the impression that the Earth is flat (or has a slight curvature).