Physics
Stars twinkle because

the intensity of light emitted by them changes with time.
the refractive index of the different layers of the earth's atmosphere ch
the distance of the stars from the earth changes with time.
None of these

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Physics
Rainbow is due to

ionisation of water deposits
absorption of sunlight in minute water droplets
diffusion of sunlight through water droplets
refraction and reflection of sunlight by water droplets

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