Physics
A star appears twinkling in the sky because of

Refraction of light by atmosphere
Reflection of light by atmosphere
Diffraction of light by atmosphere
Scattering of light by atmosphere

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Physics
RADAR is used for:

detecting and locating the position of objects such as Aeroplanes.
locating geostationary satellites.
receiving a signals in a radio receiver.
locating submerged submarines.

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