noun
periscope
An optical instrument used in submarines or armored vehicles to observe the surface or surroundings while remaining hidden.
潜水艦の艦長は潜望鏡で海上を確認した。
The submarine captain checked the sea surface through the periscope.
Compound of 潜 (せん, 'submerge, hide') + 望 (ぼう, 'look far, hope') + 鏡 (きょう, 'mirror, lens'), literally 'submerged-looking mirror'. The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning.