The Sun’s Spotlight Effect
Many globe earth believers have a hard time in believing the earth is flat as they say that is why we have night and day. They can’t figure out how we can have a night and day on a flat earth when the sun is above us. If you can illustrate how it is possible on a flat surface, it will really make the globe earth believers think.
The Sun’s area of light is limited to a circular area of light upon the earth much like the light of a lighthouse is limited to a finite circular area around it. The rotating light on a lighthouse does not propagate infinitely into the distance. This means that only certain portions of the earth is lightened at a time. It also describes how night and day arise on a flat earth. The apparent view of rising and setting are caused by perspective, just as a flock of birds overhead will descend into the horizon as they fly into the distance.
You can ask your friend to take a flash light, hold it about a foot or two above the ground at night and ask them what they see. You might want to prompt them and tell them that the light is just a circle with everything around it is dark; that light has its limits on what it can illuminate.
If you have a picture printed out like the one above, they will be able to understand it better.
Hope this helps.