Big question
Why is the sea blue?
The sea looks blue because water absorbs red light and scatters blue light back to your eyes, not because it reflects the sky. The deeper and clearer the water, the more red light gets soaked up, leaving blue behind.
Light Is Made of Colours
Sunlight looks white, but it is really a mix of all the colours of the rainbow, each one a different wavelength. When light hits water, the water molecules absorb some wavelengths and let others bounce back. Red, orange and yellow light have longer wavelengths, and water absorbs these fastest, often within the first few metres. Blue light has a shorter wavelength, so it survives much deeper before being absorbed too, and some of it bounces back out towards your eyes. That returning blue light is exactly what you see when you look at the sea, from a beach or a boat.
Not a Reflection of Sky
Many people think the sea is blue simply because it mirrors a blue sky, but that is not the real reason. You can check this yourself: the sea still looks blue on a cloudy, grey day, even though there is no blue sky to reflect. A puddle of water in a glass looks almost colourless, yet the same water in the ocean looks blue, because there is enough depth for the absorption process to happen. A calm sea surface does reflect a little bit of sky, which can add a silvery sheen, but that thin reflection is not what gives the ocean its deep blue-green colour.
Why Some Seas Look Green
Not every sea is the same shade of blue, and that is because water is rarely pure. Tiny living things called phytoplankton float near the surface and contain a green pigment called chlorophyll, which absorbs blue light and reflects green light instead. Sediment washed in from rivers, sand stirred up near a coast, and dissolved matter from decaying plants can all shift the colour too, making some coastal waters look green, brown or even reddish. The clearest, bluest water is usually found far from land, in the open ocean, where there is little sediment or plankton to change how the light bounces back.
Try it yourself
Fill a clear glass and a deep white bucket with tap water. The glass looks colourless, but look down into the bucket from above in bright light – it will show a faint blue tint, because there is now enough depth for water to absorb red light.
Questions children ask
Does the sea reflect the sky to look blue?
No, the sea's blue colour mainly comes from water absorbing red light and scattering blue light, not from reflecting the sky above it. It stays blue even under grey clouds, which proves reflection is not the main cause.
Why is the ocean sometimes green?
Ocean water turns green when it contains lots of phytoplankton, tiny organisms with a pigment called chlorophyll that reflects green light. Sediment and matter washed in from rivers near the coast can also shift the colour.
Is deep water bluer than shallow water?
Yes, deeper water usually looks a richer, darker blue because there is more distance for red, orange and yellow light to be absorbed before any light bounces back. Shallow water often looks paler or turquoise instead.