The top part of the sea is blue because of the way sunlight is scattered and reflected. Sunlight appears white but is actually made up of 7 different colours: red, orange, yellow, green, blue, indigo and violet.
When sunlight hits the water in the sea some of the colours are absorbed by the water, but because blue light is easily scattered, it gets reflected back off the water and into our eyes. This blue light coming into our eyes is what makes the sea appear blue.
What is also interesting is that the majority of the sea is actually completely dark! Light from the sun can only get through about the top 200m of the sea and oceans. If we go down deeper than this, the lack of light will just make everything appear very dark.
Hmmm katywiliam hello again, so why sea is blue?? I am sure, now you are familiar with the white light coming from sun, that it is composed of Mr. “ROY G. BIV”. So only the Blue color is not absorbed by the sea, that why it is reflected back and sea appears blue.
Secondly, also due to salt and algae (simple plant-like organisms), it appears blue.
Now my question to you Katty! Do you know who is Mr. Roy G. Biv? I am waiting for your answer.
As the others have said, a lot of the reason the sea is blue is due to a reflection of the sky. What is probably just as important and interesting is why the sea isn’t blue!
As James said the main reason it is blue is because of the water content and how it reflects light. So if you change what is inside the sea then that can change its appearance – obviously all sea is always mostly water! But you can get different algae and plant life that can make some patches of water more green – (for example some bays around Portofino in Italy). And as we all know the sea in British seaside towns can be more grey than blue – again this is linked to reflecting the light from the sky but can also be linked to the amount of disturbed sediment in the water.