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

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

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Physics
'Mirage' is an example of

Dispersion of light only
Total internal reflection of light only
Refraction and total internal reflection of light
Refraction of light only

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