Physics
Sun appears red in colour at sunrise and sunset due to

The fact that sun emits only red colour at that time
That sun comes out of the mountains
That all other colours scatter away except red
Red light having longer wave length scatters away

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Physics
Rainbow is due to

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

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