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Why does the ocean look blue?

Quick answer from Eartha

Sunlight is actually made of lots of colours mixed together. When light hits the ocean, water soaks up most of the red, orange and yellow colours, but bounces the blue light back to our eyes. That's why the sea usually looks blue, especially when it's deep and clear.

The longer story

Sunlight looks white to us, but it's really made up of every colour of the rainbow mixed together. You can see these colours split apart in a rainbow after it rains.

When sunlight shines onto the ocean, water absorbs, or soaks up, colours like red, orange and yellow more easily than it absorbs blue. The blue light bounces back up and travels to our eyes, which is why the sea often looks blue.

The deeper and clearer the water is, the more blue it can look, because there's more water for the red and orange light to be absorbed into before the blue bounces back. This is also why deep ocean water looks a richer blue than a shallow puddle.

The sea doesn't always look blue, though. Sometimes it looks green because of tiny algae living in the water, or grey and brown near river mouths where sand and mud get stirred up. The sky's colour reflecting off the surface can change the look too.

Try this at home: Spot the colours of water

  1. Fill a clear glass with water and hold it against a white background.
  2. Look at photos or videos of the ocean in different places.
  3. Talk about why the colour might look different in shallow water compared to deep water.

Words to know

Absorb
To soak up or take in something, like water soaking up colours of light.
Reflect
To bounce back, like light bouncing off water.

For grown-ups: what is my child learning here?

Age band: 8-11

Curriculum link: Australian Curriculum v9 Science, AC9S5U02 - light and its properties

The concept: Introduces basic light absorption and reflection using a familiar visual example.

More classroom ideas are on our teachers page, and safety notes for families are on for parents.

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