You are standing on a sphere with the same average diameter as the earth. This sphere has no features except the smooth surface.

Someone is walking in an absolutely straight level line away from you. You can use any handheld optical aid that takes in light and magnifies it when you look into it.

How far will the person get before they are out of sight?

Topics: travel, science, medicine, astronomy, physics, light, glass, eyes, vision, anatomy, distance, telescope