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Glass or ocean?
Look at white paper through 10 cm of water, then 50 m. When does the blue show up?
Colours computed from real pure-water absorption data.
Why is the ocean blue? The short answer
The ocean is blue because water absorbs colours in the red part of sunlight, and leaves the blue behind. NOAA says it works "like a filter".1 Sunlight is a mix of every colour, from red to violet.4 As it travels down through the sea, water soaks up the red first, then orange and yellow, then green. Blue and violet travel deepest.24 Some of that leftover blue light is scattered by the water and by tiny particles in it, and some of it comes back up to your eyes. So the water itself looks blue.25
Reflection of the sky adds a little on top, and plankton, mud and sand can shift the colour to green, brown or turquoise.25 But the main reason is the water itself.
Myth: "The ocean is only blue because it reflects the sky." This is the most common answer, and it's mostly wrong. The surface does reflect the sky, which is why the sea can look grey on a stormy day or pink at sunset. But that only matters much when the water is smooth and you look across it at a low angle.5 Swim below the surface, where no sky is reflected, and the water is still blue.2 Reflection helps a little. Absorption by the water does most of the work.18
Is water blue or clear?
Both, depending on how much you have. Even perfectly pure water is not colourless. It has a slight blue tint, but you can only see it through a long column of water.8 Chemists Charles Braun and Sergei Smirnov filled a 3-metre tube with purified water, and every one of their ten or more observers saw blue when they looked through it.7
Why can't you see it in a glass? Because the effect is very weak. Using lab measurements of pure water,10 you can work out that 10 centimetres of water lets through more than 94% of every colour. Your eyes can't spot such a small difference, so a glass or a bath looks clear. After a few metres, though, most of the red is gone, and the blue starts to show.
| Colour of light | Left after 10 cm | Left after 1 m | Left after 10 m | Depth where only 1% is left |
|---|---|---|---|---|
| Red (680 nm) | 96% | 63% | 1% | ~10 m |
| Orange (610 nm) | 97% | 77% | 7% | ~17 m |
| Yellow (580 nm) | 99% | 91% | 41% | ~50 m |
| Green (540 nm) | 99.5% | 95% | 62% | ~100 m |
| Blue (470 nm) | 99.9% | 99% | 90% | ~430 m |
Calculated from the absorption of pure water measured by Pope and Fry (1997).10 Real seawater contains particles and plankton, so light fades faster than this, and red goes even sooner. These are best-case numbers.
What makes water absorb red light?
Most coloured things get their colour from electrons soaking up light. Water is different. A water molecule is one oxygen atom holding two hydrogen atoms, and the bonds between them act like tiny springs that are always vibrating. Red light carries just the right amount of energy to make those springs vibrate harder, so it gets absorbed. Blue light doesn't match, so it slips through.78
Braun and Smirnov point out that water may be the only thing in nature whose colour comes from vibrating molecules like this.7 They also tested "heavy water", where the hydrogen atoms are heavier. Its springs vibrate more slowly, so they don't absorb visible red light, and heavy water is colourless even over metres.7 Water absorbs the rest of the visible colours only very weakly, especially blue. That's why it's so transparent in the first place.9
Why does red disappear when you dive?
Take something bright red underwater and it looks less and less red the deeper you go, because there's less red light left for it to reflect.6 In our dive toy, a red stripe on the diver's wetsuit turns a dull grey at about 10 to 15 metres. That figure comes from pure-water data; in murkier seawater red fades out even sooner.210 WHOI says that more than a few metres down, most red and orange light has vanished. Yellow and green go next, and blue and violet reach deepest.2
This is why so many deep-sea animals are red. At 100 metres no red light gets through at all, so a red fish looks blackish and is almost impossible to see.4 We only see their red colour in photos because white lights on submersibles and cameras shine on them.6
The light also gets dimmer overall. In clear ocean water it drops about ten times for every 75 metres you go down.6 The top 200 metres is the sunlight zone. Below that there's only a faint glow, the twilight zone, and below about 1,000 metres no sunlight reaches at all (only the light some animals make themselves).13 So most of the ocean is too deep for sunlight to reach, and it's dark down there.
Sky vs sea: two different blues
The sky and the sea are both blue, but for different reasons. The sky is blue because air molecules scatter short blue light waves all over the sky. The ocean is blue mainly because water absorbs red light.8 The USGS spells it out: water's blueness "is not caused by the scattering of light, which is responsible for the sky being blue".8 Scattering still plays a part in the sea. It's what sends the leftover blue light back up to your eyes.25 And neither gets its main blue from reflecting the other.
| Sky | Ocean | |
|---|---|---|
| Main reason it's blue | Scattering: air molecules bounce blue light around | Absorption: water molecules soak up red light |
| What happens to red light | Mostly passes straight through the air | Gets used up within metres |
| Changes colour when… | The Sun is low (sunsets) | There's plankton, mud or shallow sand |
| Role of reflection | None | A little: the surface mirrors the sky at low angles |
What colour is the ocean? Green, brown, turquoise…
The sea isn't always blue. Anything floating or dissolved in the water changes which colours survive.18
- Green: near coasts the water often has lots of phytoplankton. These are tiny floating algae that contain chlorophyll, just like plants.11 Chlorophyll absorbs some colours and reflects others, so plankton-rich water looks green.25 (Plants use chlorophyll to catch light too. See how photosynthesis works.)
- Brown: rivers carry silt and sand into the sea, and storms stir sediment up from the bottom. These particles reflect longer wavelengths and turn the water brown.25 Dissolved plant matter can also make water yellow or brown.8
- Turquoise: in shallow, clear water over white sand, like parts of the Caribbean and the Bahamas, sunlight reaches the bottom and bounces back up. Only a few metres of water means some green light survives with the blue.2
- Red or milky: some algae blooms tint the sea reddish. The chalky scales of tiny coccolithophores can turn it milky white or bright blue.812
Scientists use these colour changes to measure the amount of phytoplankton in the ocean from satellites.12 NASA's PACE satellite is designed to go further and tell different kinds of plankton apart.13
About the toys: the colours for pure water and for depth come from real lab data for water.10 The amounts of plankton, mud and sand in the "kind of water" and "mix your own ocean" toys use a simplified model with made-up but realistic numbers, so they show the trend, not a measured colour of any real sea.
Try it at home
You won't see water's own blue in a glass, but you can see how depth changes things. Next time you're at a pool or a clear lake, look at a white tile or sandy bottom at the shallow end, then at the deep end. The deep end looks bluer. That's more water between you and the bottom soaking up more red. Or look at a photo taken by a diver without a flash: in clear water, red things look much darker (brownish or grey) by about 15 metres.6
Common misconceptions
"The sea is blue for the same reason as the sky." The sky is blue because of scattering by air. The sea is blue mainly because water absorbs red.8
"The ocean is always blue." It can be green, brown, turquoise or even red, depending on plankton, sediment and depth.212
"A red fish would stand out in the deep sea." Where there's no red light, red looks black. That's why red is a common hiding colour down there.4
Keep exploring
Why does that blue water taste so salty? Read why the ocean is salty. Curious about the surface? See what causes waves. And for more on light itself, try how light travels, or see how sunlight splits into its colours in how rainbows form.
FAQ
Why is the ocean blue, in simple words?
Water soaks up red, orange and yellow light a little more than blue. In a glass you can't notice it, but after several metres of water most of the red is gone. The blue light that's left bounces around in the water and some of it comes back to your eyes, so the sea looks blue.
Why is the ocean blue but bath water is clear?
The effect is very weak. Ten centimetres of water lets more than 94% of every colour through, so a glass or a bath looks clear. It takes metres of water to soak up enough red for the blue to show, and the ocean is thousands of metres deep.
Is the ocean blue because it reflects the sky?
Only a little. The surface does reflect the sky, mostly when the water is calm and you look across it at a low angle, which is why the sea can look grey on a cloudy day. But divers under the surface still see blue water, where no sky is reflected. The main reason is that water absorbs red light.
What is the true colour of ocean water?
Pure water is very pale blue. You only notice it through a long column, like metres of water. Real seawater can also be green, brown, turquoise or even reddish, depending on plankton, mud and how deep the sea floor is.
Why is the ocean green in some places?
Green water usually means lots of phytoplankton, tiny floating algae that contain green chlorophyll. Chlorophyll soaks up blue light, so together with the water it leaves mostly green. Mud and plant matter from rivers can also turn the water green or brown.
Why is the water in the Bahamas and Caribbean so turquoise?
The water there is very clear and shallow, over white sand. Sunlight reaches the sandy bottom and bounces back up. With only a few metres of water on the way, some green light survives along with the blue, giving that bright turquoise.
Is the sky blue because of the ocean?
No, and the ocean isn't just reflecting the sky either. The sky is blue because air molecules scatter blue light across the sky. The ocean is blue mainly because water absorbs red light.
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Sources
- NOAA National Ocean Service, Why is the ocean blue?
- Woods Hole Oceanographic Institution, Why is the ocean blue?
- NOAA National Ocean Service, How far does light travel in the ocean?
- NOAA Ocean Exploration, Why are so many deep-sea animals red?
- Library of Congress, Everyday Mysteries: Why is the ocean blue?
- E. Widder, Deep Light (NOAA Ocean Exploration)
- C. L. Braun & S. N. Smirnov, Why is water blue? Journal of Chemical Education 70, 612 (1993)
- USGS Water Science School, Water color
- HyperPhysics (Georgia State University), Transparency of water in the visible range
- R. M. Pope & E. S. Fry, Absorption spectrum (380–700 nm) of pure water, Applied Optics 36, 8710 (1997)
- NOAA National Ocean Service, What are phytoplankton?
- NASA Earth Observatory, What are phytoplankton?
- NASA PACE mission