Physics
The optical phenomenon that is primarily responsible for the observation of rainbow on a rainy day is

Dispersion
Reflection
Interference
Diffraction

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

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