Physics
Radiocarbon is produced in the atmosphere as a result of

Collision between fast neutrons and nitrogen nuclei present in the atmosphere.
Action of ultraviolet light from the sun on atmospheric oxygen.
Lightning discharge in atmosphere.
Action of solar radiations particularly cosmic rays on carbon dioxide present in the atmosphere.

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Physics
Planets do not twinkle because

They are very far away from the earth resulting in decrease in intensity of light
Their distance from the earth does not change with time
They are nearer to earth and hence we receive a greater amount of light.
They emit light of a constant intensity

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