NCERT Class 11 Physics Solutions: Chapter 2-Unit and Measurement Part 15 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question: 2.29

A LASER is a source of very intense, monochromatic, and unidirectional beam of light. These properties of a laser light can be exploited to measure long distances. The distance of the Moon from the Earth has been already determined very precisely using a laser as a source of light. A laser light beamed at the Moon takes 2.56 s to return after reflection at the Moon՚s surface. How much is the radium of the lunar orbit around the Earth?

Answer: 2.29

  • Time taken by the laser beam to return to Earth after reflection from the Moon = 2.56 s
  • Speed of light
  • Time taken by the laser beam to reach Moon
  • Radius of the lunar orbit = Distance between the Earth and the Moon

Question: 2.30

A SONAR (sound navigation and ranging) uses ultrasonic waves to detect and locate objects under water. In a submarine equipped with a SONAR the time delay between generation of a probe wave and the reception of its echo after reflection from an enemy submarine is found to be 77.0 s. What is the distance of the enemy submarine? (Speed of sound in water = 1450 m ) .

Answer: 2.30

  • Let the distance between the ship and the enemy submarine be ‘S’ .
  • Speed of sound in water = 1450 m/s
  • Time lag between transmission and reception of Sonar waves = 77 s
  • In this time lag, sound waves travel a distance which is twice the distance between the ship and the submarine (2s) .
  • Time takes for the sound to reach the submarine
  • Distance between the ship and the submarine (S)

Question: 2.31

The farthest objects in our Universe discovered by modern astronomers are so distant that light emitted by them takes billions of years to reach the Earth. These objects (known as quasars) have many puzzling features, which have not yet been satisfactorily explained. What is the distance in km of a quasar from which light takes 3.0 billion years to reach us?

Answer: 2.31

  • Time taken by quasar light to reach Earth = 3 billion years

  • Speed of light
  • Distance between the Earth and quasar

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