Why Is the Ocean Blue? A Colour Made by Vibrating Molecules

C.V. Raman carried a Nicol prism in his pocket when he sailed home from Europe in September 1921, and from the SS Narkunda he turned it on the sea. Held at the correct angle to the water and turned the right way, the prism quenches the sky’s reflection on the surface.

Lord Rayleigh, the physicist who had explained why the sky is blue, had written in 1910 that the deep sea’s dark blue “has nothing to do with the colour of water, but is simply the blue of the sky seen by reflection”. On that account, removing the reflection should have removed the blue.

Looking through the prism at “the deeper waters of the Mediterranean and Red Seas”, Raman found the blue undimmed. It was, he told Nature in a letter printed on 17 November 1921, “a distinct phenomenon in itself, and not merely an effect due to reflected sky-light”.

Why is the ocean blue?

The ocean is blue because water absorbs red light. Water molecules vibrate, and high overtones of those vibrations absorb the red end of sunlight, so every metre of water removes more red than blue. Scattering, by the water itself and by particles in it, turns some of that filtered light back toward the surface, and it arrives blue.

Reflected sky adds to the view from above, but it cannot explain the blue underwater, where, as Woods Hole Oceanographic Institution notes, the colour remains with no sky to reflect. The vibration is the rare part: Dartmouth chemists Charles Braun and Sergei Smirnov knew of no other colour in nature made that way.

Red light is spent in the first few metres

In a 1997 paper, Pope and Fry, at Texas A&M, reported pure water’s absorption from 380 to 700 nm and put the minimum at 418 nm, near the violet end of the spectrum, at 0.0044 per metre. At 700 nm, deep red, their data give 0.624 per metre, about 140 times stronger.

Light	Wavelength	Absorbed in 1 m	Left after 10 m
Violet	418 nm	0.4%	96%
Blue	450 nm	0.9%	91%
Green	550 nm	5.5%	57%
Red	650 nm	29%	3%
Deep red	700 nm	46%	0.2%

Percentages are calculated from Pope and Fry’s coefficients for pure water.

Blue goes much further, though NOAA says no light penetrates deeper than 3,280 feet (1,000 m). A glass of water looks clear because its path is too short: a 10 cm glass loses only about 3 percent of its red.

Veins work differently. Skin over a vein still returns more red than blue, and reads as blue only beside the redder skin around it.

Water’s blue comes from vibration, a rare source of colour

Water’s O-H bonds vibrate at infrared wavelengths near 3 micrometres, Physics Today says, far too low in energy to absorb visible light one quantum at a time. Absorbing red takes an overtone, four or five quanta of stretching at once.

In their 1993 paper, “Why is water blue?”, Braun and Smirnov placed one four-quantum overtone in water vapour at 698 nm, “just at the red edge of the visible spectrum”. Hydrogen bonding in the liquid shifts the bands to lower energy and longer wavelengths, so liquid water’s visible absorption is the short-wavelength tail of a band centred at 760 nm, plus weaker bands at 660 and 605 nm.

Each extra quantum makes a transition 10 to 20 times weaker, so the absorption is faint and fades toward the blue, which would need still more quanta.

“To our knowledge the intrinsic blueness of water is the only example from nature in which color originates from vibrational transitions,” Braun and Smirnov wrote.

Physics Today is more guarded, calling electron interactions “the primary determinants of color in almost all other substances”. A mood ring makes its colour by reflection instead: the pitch of a liquid crystal’s helix sets which band of wavelengths bounces back, and warming shortens the pitch, moving the colour toward blue.

Heavier hydrogen takes the colour out of water

Swap ordinary hydrogen for deuterium, its heavier isotope, and you have heavy water, the kind whose ice sinks in a glass of ordinary water. Braun and Smirnov’s spectrum shows the 760 nm band moving to about 1,000 nm, in the infrared, where the eye cannot see it. “D2O is colorless,” they wrote, “because all of its corresponding vibrational transitions are shifted to lower energy by the increase in isotope mass.”

Their eyeball test, a 3 m tube of purified ordinary water hung over sunlit white paper, had ten or more observers reporting blue. They never filled it with heavy water, because “the large tube volume and a limited budget precluded checking”.

Green, turquoise and brown come from what the water carries

Chlorophyll in phytoplankton absorbs blue and red, so as their numbers rise the sea sends back less blue relative to green, Physics Today says, and NASA’s Earth Observatory says blooms can turn water “greenish, reddish, or brownish”. Silt and sand turn it brown, and clear shallows over sand look turquoise because light reaches the bottom and bounces back, Woods Hole says.

The clearest water leans toward violet, the colour pure water absorbs least: Physics Today reports that “in some very clear, clean locations, most notably near Easter Island in the South Pacific Gyre, the water appears almost purple”.

Rayleigh’s own version was subtler than the myth

The reflected-sky answer is a myth, but Rayleigh’s 1910 piece in Nature, written after a “voyage round Africa”, was more careful than the slogan. He granted water an “absorption, or proper, colour”, but argued that in clear, deep ocean “there is often nothing to send the light back to the observer”. He even answered people who found the sea “purer and fuller” than the sky: they were comparing it with the sky near the horizon, when the best blue is overhead.

Raman turned that observation round. The water’s hue, he wrote, was “of such fullness and saturation that the bluest sky in comparison with it seems a dull grey”. Rayleigh never read the reply: he had died on 30 June 1919.

Raman’s scattering was half the answer

The other common answer, that the sea is blue for the same reason as the sky, gets one ingredient right. That was Raman’s own explanation: it “seemed not unlikely”, he told his Nobel audience, “that the phenomenon owed its origin to the scattering of sunlight by the molecules of the water”.

Molecular scattering does favour blue, in water as in air, but on its own it is not the answer. Physics Today credits a complete explanation to Vasily Shuleikin, whose 1923 paper combined the absorption Robert Bunsen had argued for with scattering by fine particles and bubbles, reflection from larger particles, and reflected sky.

The scattering work still led Raman, in 1928, to the effect named after him, in which a small fraction of scattered light emerges at a changed wavelength, and to the 1930 Nobel Prize in Physics. The first section of his Nobel lecture is headed “The colour of the sea”.

In water, the effect that carries his name comes from vibrations of the water molecule, Physics Today notes, the same motions whose overtones take the red out of sunlight. Raman went looking for a scattering explanation of the blue and found a scattering effect driven by the vibrations that cause it. Strictly FYI.