ï»¿ A LASER is a source of very intense, monochromatic, and unidirectional beam ofÂ 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 preciselyÂ using a laser as a source of light. A laser light beamed at the Moon takes 2.56 s toÂ return after reflection at the Moonâ€™s surface. How much is the radius of the lunarÂ orbit around the Earth? from Physics Units and Measurement Class 11 Manipur Board

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A LASER is a source of very intense, monochromatic, and unidirectional beam ofÂ 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 preciselyÂ using a laser as a source of light. A laser light beamed at the Moon takes 2.56 s toÂ return after reflection at the Moonâ€™s surface. How much is the radius of the lunarÂ orbit around the Earth?

Radius of theÂ lunar orbit is the distance between the earth and the moon.Â

Laser beam takes 2.56 second to reflect back, therefore distance of the moon from the earth is given by,Â

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What is the need of measurement?

To get the complete knowledge of any physical quantity, measurement is needed. Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit. Knowledge without measurement is incomplete and unsatisfactory.
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What is measurement?

Measurement is the process of attaching a numeric value to an aspect of a natural phenomenon. Measurement is making the quantitative knowledge of a physical quantity.
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Define unit.

The quantity used as a standard of measurement is called unit.

Example: Unit of time is second.

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Give four examples of physical quantities.

Examples of Physical quantitie are force, mass, density and power.
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Define physical quantity.

A measurable quantity in terms of which laws of Physics can be expressed is called physical quantity.

The result of a measurement of a physical quantity is expressed by a number accompanied by a unit.

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