A periscope is an optical instrument that allows submarine operators to visually observe the situation above the surface while submerged. Like other optical devices, Earth’s curvature can obscure the visibility of a distant object.
Flat-Earthers claim a periscope can see objects that are too far, and they use it to “prove” a flat Earth. In reality, everything is consistent with spherical Earth. A ship 50 m (165 ft) high at a distance of 25 km (15.5 miles) can still be visible if the weather allows & the periscope is 2 m (7 ft) above the surface.