Physics
Rainbow is due to

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

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Physics
In a railway track, two rails are joined end to end with a gap in between them because

Accidents due to expansion in summer can be avoided
Air-gaps are necessary for bearing the weight of running train
Steel can be saved
Accidents due to contraction in winter can be avoided

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