Physics
Light houses are places with powerful lights to :

Guide and warn the ships coming from different directions in the ocean
Indicate to the incoming warships, the location of a harbour during night
Guide and help large crowds at religious gathering during nights
Guide and resolve traffic jams in crowded metro-cities during nights

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Physics
It is easier for a man to swim in sea water, than in river water because

The density of sea water is lesser than the density of river water
The sea is wider than the river
The sea is-deeper than the river
The density of sea water is more than the density of river water

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