Which instrument is used in submarines to ovserve the surface of water?

The instrument used in submarines to observe the surface of water is called a periscope. A periscope consists of a long tube with mirrors at each end, allowing the viewer to see objects above the water's surface while the submarine remains submerged. The periscope's mirrors reflect light from the surface down the tube to the viewer, providing a view of the surroundings without the need to surface the submarine.