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Why does the sea sometimes look bright blue, and sometimes green?

The sea looks blue or green because of tiny algae, sand, and bits of soil in the water, and the way light hits the water.

Sea water is really colourless, just like a glass of water. Sunlight looks white, but it actually holds all the colours inside it. Water swallows up colours like red and orange very quickly, but it bounces blue back out. That is why deep, clean seas look bright blue to us.

In some seas, tiny algae and tiny living things float in the water. These tiny algae carry a green substance, and they feed using sunlight. When lots of them grow, the water looks like it has been painted green.

Near the coast, sand and soil mix into the water too. This changes the colour as well. If the sky is cloudy, the sea can look greyer or paler, because the sea also reflects a bit of the sky's colour.

Quick facts

Sea water is actually colourless on its own.
Water quickly swallows up red light, but bounces blue light back out.
Tiny living things in the sea are called plankton, and some of them are green.
When plankton grow in huge numbers, the sea water can look green.
When sand and soil near the coast mix into the water, the sea's colour changes.
When the sky is cloudy, the sea can look paler too.

Check what you remember

4 questions — all of them come from the quick facts above. There is no score and no wrong turn.

1. What is sea water like on its own?

2. Which colour does water swallow up quickly?

3. What are tiny green living things in the sea called?

4. What can change the sea's colour near the coast?

Where this comes from

1 tr.wikipedia.org 80/100
Sea - Wikipedia
Explains simply what the sea is and its features.
2 tr.wikipedia.org 75/100
Plankton - Wikipedia
Gives information about the tiny living things that turn the sea green.
3 nasa.gov 70/100
NASA
Has information showing the colour of the oceans from photos taken in space.

🌈

Light and Colours

Sunlight looks white, but it holds all the colours of the rainbow inside it. Water swallows up red quickly, but it throws blue back out.

🌿

Sea Algae

Very tiny living things in the sea are called plankton. Some plankton are green, and when they grow in huge numbers, they turn the water green.

☁️

The Colour of the Sky

The sky itself looks blue because sunlight spreads out in the air. The sea also reflects a little bit of the sky's colour.

🏖️

The Coast and Sand

Near the coast, sand and soil mix into the water. This mix can turn the sea green or brown.

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