Why Is the Sea Blue?
A Completely Different Reason From the Sky's Blue
On a clear day, the sea shines blue. You might think it is just the blue of the sky reflected on the surface. But the sea looks bluish even on cloudy days, and it has been confirmed that water itself has a faint blue-green tint. The blue of the sea comes from a completely different mechanism from the blue of the sky.
Imagine filling a big white bathtub with plenty of water. A small amount looks almost colourless and clear. But at the depth and volume of a swimming pool, the water itself can look faintly blue-green.
It isn't reflecting the sky, and nothing has been dissolved in it. Simply gathering a lot of water in one place makes it look coloured.
Water molecules are thought to absorb a little red light (light with a long wavelength) but very little blue light (light with a short wavelength).
The farther light travels through water, the red light is absorbed and disappears first, and the surviving blue light is scattered and reaches our eyes, it is thought.
Let's look at "absorption by water itself" and "colour change with depth", one at a time.
Water itself absorbs a little red light
Water molecules absorb slightly different amounts of light depending on its colour (wavelength). They are thought to absorb a little more of long-wavelength light, such as red and orange, than of short-wavelength light, such as blue. In a glass of water, the difference is tiny and you won't notice it. But when the amount of water (the distance the light travels) becomes large, as in a pool or the sea, this small difference builds up into a clear difference in colour.
The deeper the light goes, the more red vanishes and only blue remains
Sunlight is a mix of many colours (wavelengths), from red to violet. When it enters water, the long-wavelength red light is absorbed first, and it has almost entirely vanished within a few metres of depth. Orange and yellow, with shorter wavelengths, reach a little deeper. Only the blue light, with the shortest wavelength of these, reaches relatively deep. It hits water molecules and tiny particles in the water and is scattered, and some of it comes back to the surface. What reaches our eyes is this "blue light that survived to the end".
If the sea's blue were only a reflection of the sky, the sea should look grey on cloudy days. In reality, the sea keeps a faint blue tint even under cloud. Also, water takes on a blue-green tint even in places with little sky reflection, such as a swimming pool. These are thought to be clues that the blue of the sea comes from the properties of water itself.
It is blue because water itself swallows red light selectively.
Why does the colour differ from place to place?
Clear open ocean is a deep blue, while coastal seas often look green or brownish. This is thought to be because phytoplankton in the water (which contain a green pigment called chlorophyll) and grains of sand and mud carried by rivers affect how light is absorbed and scattered, separately from the water itself. Waters rich in plankton look greener, and muddy coastal waters look browner.
Things you can check for yourself
- Fill a large white container (a bucket or bathtub) with water, as deep as you can
- With a white sheet of paper or a tile visible at the bottom, look down through the water from directly above
- Compare the shallow and deep parts to see whether the colour differs
- If you can, also compare the colour of the shallows and the deeper water in a pool or the sea
You can get a feel for how, as the water gets deeper, the light travels farther and a growing share of the red light is absorbed.
Summary
The sea is blue not because it reflects the colour of the sky, but because water itself absorbs a little red light and very little blue light, it is thought. The deeper light travels in water, the more the red light is absorbed and vanishes, colour by colour, and the blue light that survives to the end is scattered and reaches our eyes. Though both are "blue", the sky's blue arises from a completely different mechanism.
The sky's blue is the result of light being "scattered".
The sea's blue is the result of light being "picked out and left standing".
How light splits into colours by wavelength is also explained in the article on the rainbow after rain. For the mechanism behind the blue of the sky itself, see this article. If you are more curious about the sea's taste than its colour, we also recommend Why Is Seawater Salty?
What it feels like for a tiny creature to swim in that water is covered in Why Is Water as Sticky as Honey to a Water Flea?
For those who want to know more: terms, numbers and links to textbooksFrom middle-school science to topics under active research, with the level of each part clearly marked
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school basic physics
- High school+Advanced or sidebar content in high-school textbooks
- UniversityUniversity-level specialist content (oceanic physics) not taught in high school
- ResearchTopics researchers are still investigating, not yet settled enough to be taught even at university
Middle schoolTerms: words about the colour of the sea
- Absorption: when a material takes in the energy of light, so light of that colour becomes weaker.
- Scattering: when light hits particles or molecules and sets off in new directions.
- Wavelength: the length of one repeat of the light wave, which sets its colour. Red is long; blue is short.
High schoolChecking with a formula: how differently do red and blue survive at 10 m deep?
In water, light is thought to be absorbed at a constant rate for each extra metre of depth and so grows weaker. We use this "constant rate of weakening" to compare how red and blue light survive.
Remaining light intensity = starting intensity × (survival rate per 1 m of depth) raised to the power of the depth
| Survival rate of red light | 60% per metre (a rough value) |
| Survival rate of blue light | 95% per metre (a rough value) |
* Real survival rates vary a great deal with the sea area and the clarity of the water. Here we use rough numbers to make the difference between red and blue easy to picture.
| Red light (0.6 to the power of 10) | 0.6 × 0.6 × … (10 times) = about 0.006 (0.6%) |
| Blue light (0.95 to the power of 10) | 0.95 × 0.95 × … (10 times) = about 0.60 (60%) |
| Result | At 10 m deep, red light has almost vanished, while about 60% of the blue light is still left |
After travelling the same 10 m, red light falls to less than 1% of its starting strength, while more than half of the blue light is still there. This difference, built up over distance, is thought to be why deep water looks blue. It is also thought to be why, for a diver, red things (such as their own blood) look blackish at depth: the red light has been absorbed.
High school+Why does water absorb red light so easily?
When a water molecule (H₂O) absorbs light, the bonds inside the molecule start to stretch and contract slightly. The energy at which this vibration easily occurs (a component called an overtone of the vibration) happens to be close to the energy of red light, so red light is thought to be absorbed more easily than blue. The faint colour of pure water comes from this property of the molecule.
UniversityHow oceanic physics classifies the colour of the sea
In oceanic physics, researchers distinguish clear open-ocean water, whose colour is set by the absorption properties of water itself (Case 1 waters), from coastal water strongly affected by plankton and sediment (Case 2 waters). The technique of "ocean colour remote sensing", which estimates chlorophyll concentration and water quality by observing the colour of the sea from satellites, is also based on this idea.
ResearchWhat is still unclear
- Very precise values of water's absorption spectrum (how much of each wavelength it absorbs) are thought to change slightly with temperature, pressure and dissolved substances, and precise measurements under extreme conditions such as the deep sea are still being made.
- The technique of telling apart even the types of plankton from satellite ocean-colour observations is a research field where accuracy is still being improved.
- Research is also under way that tracks changes in ocean colour over long periods to study how climate change is affecting marine ecosystems.
Even the blueness of the sea holds a rich theme where the physics of light meets ocean science, and research on it continues today.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of light | Basic terms: absorption, scattering, wavelength |
| High school | Basic physics: waves (advanced) | Calculating light survival by depth |
| High school+ | Chemistry: molecular vibration (advanced) | How water molecule vibration relates to red absorption |
| University | Oceanic physics | Case 1 / Case 2 waters, ocean colour remote sensing |
| Research | Ocean science (under research) | Precise absorption spectrum measurements, telling plankton species apart, links to climate change |
- Explanations of the optical properties of water and ocean colour in oceanic physics textbooks.
- Explanations of how water molecules absorb red light, in optics and spectroscopy materials.
- Explanations of satellite ocean-colour observation and chlorophyll estimation, in remote sensing materials.
* The light survival figures are rough values for understanding the mechanism. Real values are said to vary a great deal with the sea area and the clarity of the water.
* This article is a science explainer for general readers. The figures given are rough estimates to help you understand the mechanism. Real values are said to differ greatly by sea area and conditions.