Question

why is the sea water blue in colour​

Answer 1

When sunlight hits a sea or an ocean, the water strongly absorbs long-wavelength colors at the red end of the light spectrum, as well as short-wavelength light, including violet and ultraviolet.

The remaining light that we see is mostly made up of blue wavelengths.

So , The blue hue of water is an intrinsic property and is caused by the selective absorption and scattering of white light.

Note :

         This effect only works if the water is very pure; if the water is full of mud, algae or other impurities, the light scattered off these impurities will overwhelm the water's natural blueness.

Hope this may help you ,

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Answer 2

In water, absorption is strong in red and weak in blue ,thus red light is absorbed quickly in ocean leaving blue . Almost all sunlight that enters the oceans is absorbed.