Which of the following is the instrument used in submarines to view objects above sea level?

Periscope

The correct answer is option 4. Periscope.

A periscope is a device used on submarines to observe objects above the water's surface while the submarine remains submerged. It typically consists of a long tube with mirrors positioned at both ends. One mirror, called the top mirror, is located above the waterline and reflects light down the tube. The other mirror, known as the bottom mirror, is angled to reflect the light horizontally, allowing the observer to view objects outside the submarine without having to surface.

When light enters the periscope through the top mirror, it travels down the tube and is reflected by the bottom mirror. This reflection allows the observer to see a magnified image of objects above the waterline. By rotating and adjusting the periscope, the observer can scan the surroundings and gather visual information without the need to expose the submarine.

Periscopes play a crucial role in submarine operations, providing valuable situational awareness and aiding navigation, especially when the submarine needs to remain hidden or operate stealthily beneath the surface. They allow submariners to monitor surface vessels, landmarks, and potential threats, enhancing the submarine's ability to navigate safely and effectively in various environments.