Physics
Purpose of an optical filter is to

Dispense light into component colours
Reflect lights of different colours
Refract light of different colours
Transmit or absorb light of different colours

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

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