What is a Periscope?

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About Periscope:

It is an optical instrument used in land and sea warfare, submarine navigation, and elsewhere to enable an observer to see his surroundings while remaining undercover, behind armor, or submerged.
Submarines have periscopes so that people inside can see what is on the surface of the water.

Construction and Working Principle:

Periscopes can be constructed using a pair of flat mirrors or prisms and a rectangular hollow tube that has a viewing window at the top and bottom of the tube.
The mirrors within this structure are fixed at an angle of 45o at the two ends of the rectangular hollow.
A periscope is a useful example of the law of reflection at work. It’s important that your mirrors are placed at a 45° angle, because light always reflects away from a mirror at the same angle that it hits it.
In a periscope, light from an object strikes the top mirror at 45° and bounces off at the same angle.
This sends light directly down the tube and onto the lower mirror. This mirror, also at a 45° angle, reflects light directly to your eye.
A simple periscope only requires mirrors, whereas a more complicated periscope utilizes prisms rather than mirrors.
The periscopes used in submarine applications are complex, typically requiring the use of tubes that are at least 10 meters in length to achieve a greater range of scope.

Q1) What is the Central Scientific Instruments Organisation (CSIO)?

CSIO Chandigarh, is a constituent laboratory of the Council of Scientific & Industrial Research (CSIR), a national laboratory dedicated to research, design and development of scientific and industrial instruments. It is a multi-disciplinary and multi-dimensional apex industrial research & development organization in the country to stimulate growth of the Instrument Industry in India covering a wide range of applications.

Source: In a first, Navy submarine periscope refurbished at CSIO