A sketchbook explainer · 5 min
Explained for ages 8–12
Your turn
Each lab is one part of the video. Tap and slide, and see what happens.
Each lab isolates one step. Colours come from real pure-water absorption data; plankton, mud and sand amounts are a simplified model.
Tap the buttons and slide the sliders.
Look at white paper through 10 cm of water, then 50 m. When does the blue show up?
Transmitted colour of white light after a path through pure water, 0.1 to 50 m (Pope & Fry data).
Slide from a little water to a lot. What colour do you see?
Colours computed from real pure-water absorption data.
Tap a colour and send it down. How far does it get before only 1% is left?
Depth at which 1% of each wavelength remains in pure water (Beer–Lambert).
Tap a colour. Which one goes deepest?
Clearest, purest water; real seawater stops red even sooner.
Drag the depth. When does the red stripe turn grey?
Light left per band, and the apparent colour of a red stripe, from 0 to 200 m.
Go down deep. What happens to the red stripe?
Pure water plus a little from particles (a rule of thumb).
Change the weather. Does the view through the glass floor change?
Surface reflection changes with the sky; the upwelling colour below the surface does not.
Change the weather. Is the water under the boat still blue?
Pick a kind of water. What makes it green, brown or turquoise?
Modelled ocean colour for open ocean, plankton-rich coast, a river mouth and a sandy lagoon.
Simplified colour model.
Mix plankton, mud and a sandy floor. Can you make turquoise? Brown? Green?
Combine chlorophyll, sediment and bottom depth in a simplified reflectance model.
Add tiny plants, mud and sand. What colour is your sea?
Real water data + made-up plankton/mud amounts.
Fill a glass with sea water. It looks clear! But out on a boat, the sea looks deep blue. How can that be?
Sunlight looks white, but it has all the colours of the rainbow in it.
Water drinks up a little bit of light. It drinks red the fastest. A glass is too small to see it. But the sea is very, very deep. Down there, the red is all gone, and blue is left.
The blue light bounces around in the water. Some comes back up to your eyes. That's the blue you see!
Some people think the sea copies the sky. The top shines a bit, and looks grey on a cloudy day. But under the water, it is still blue.
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.
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.
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 is also clear for most other colours of visible light because there's nothing else in the molecule that can absorb there. That's why it's so transparent in the first place.9
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 submarines shine bright white lights 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 1,000 metres it's completely dark.13 So most of the ocean isn't blue at all. It's black.
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 one is a reflection of 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 |
The sea isn't always blue. Anything floating or dissolved in the water changes which colours survive.18
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.
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: anything red at 15 metres looks brown or grey.6
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.
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.
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.
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.
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.
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.
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.
No, it's the other way around: neither is a reflection of the other. The sky is blue because air molecules scatter blue light across the sky. The ocean is blue mainly because water absorbs red light.