Physics
Purpose of an optical filter is to

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

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Physics
The blue colour of water in the sea is due to

Reflection of blue light by impurities in sea water
Absorption of other colours except blue by water molecules
Scattering of blue light by water molecules
Reflection of blue sky by sea water

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