Nothing is as mesmerising as the ocean’s blue colour - but what makes it blue? It’s physics of course, and the interaction between sunlight and Earthly matter. This is a brief story of the origins of the ocean blue.
Seen from space, light from the Sun is white. When it hits our atmosphere and the oceans beneath it, different colours begin to behave differently - blue light is scattered and red light absorbed, to a small degree. This difference in behaviour between colours is due to their different wavelengths and is formally described by a process called Rayleigh scattering. The wavelength with the highest scattering rate is around the visible spectrum, more precisely at 418nm. This specific wavelength is seen by humans as the colour, you’ve guessed it, blue.
Other colours, such as green, yellow and red, have progressively lower scattering and higher absorption rates. Going beyond the visible spectrum, into either the shortwave ultraviolet or longwave infrared, absorption rates rocket to an amazing 10,000,000 times higher than at their blue minimum. The ocean’s surface layer is practically blocking those wavelengths.
You may wonder, then, why water is tint-free in our everyday lives – a blue glass of water would definitely cause upset. The reason is that Rayleigh scattering needs larger quantities to really become evident. The more water is added, the bluer it will look.
Curiously, an ocean of the exact same chemical composition as one on Earth, but located on some far-away planet can be a completely different colour, such as violet. It is known, as per Rayleigh’s phenomenon, that the colour violet scatters even better than blue. But we (sadly!) do not have violet oceans because (1) human eyes aren’t great at detecting violet, (2) it forms only a minor part of our Sun’s spectrum and (3) it can be absorbed by the ozone layer.
Back to the blue ocean, another reason for its colour is that it simply reflects the blue sky above it. This is a straightforward explanation, but only leads us to wonder why is the sky blue? This is a different question with the same answer - Rayleigh scattering. As sunlight enters our atmosphere the blue wavelengths scatter most freely, bounce around and colour the sky, while yellow and red light travel more directly to us, thus giving the Sun has its distinctive yellow-red colour. This effect becomes more evident as the Sun is closer to the horizon. At that point the light has to travel through even more water vapour, pollution and dust to reach us, thus exaggerating the effect and causing beautiful red sunsets.
2. Nothing is as mesmerising as the ocean’s
blue colour - but what makes it blue?
3. SEEN FROM SPACE, LIGHT FROM
THE SUN IS WHITE
BUT WHEN IT HITS OUR
ATMOSPHERE AND THE OCEANS
BENEATH IT, DIFFERENT
COLOURS BEGIN TO BEHAVE
DIFFERENTLY
4. IN THE ATMOSPHERE, BLUE LIGHT
IS SCATTERED AND RED LIGHT
ABSORBED,
TO A SMALL DEGREE
A PROCESS CALLED
RAYLEIGH
SCATTERING
LORD RAYLEIGH
5. THE WAVELENGTH WITH THE
HIGHEST SCATTERING RATE IS
AROUND THE VISIBLE SPECTRUM,
MORE PRECISELY AT 418NM
THIS SPECIFIC WAVELENGTH IS
SEEN BY HUMANS AS THE
COLOUR BLUE
6. OTHER COLOURS, SUCH AS
GREEN, YELLOW AND
RED, HAVE PROGRESSIVELY
LOWER SCATTERING AND HIGHER
ABSORPTION RATES.
7. GOING BEYOND THE VISIBLE
SPECTRUM, INTO SHORTWAVE
ULTRAVIOLET OR LONGWAVE INFRARED,
ABSORPTION RATES ARE 10,000,000 TIMES
HIGHER.
THE OCEAN’S SURFACE LAYER IS PRACTICALLY
BLOCKING THOSE WAVELENGTHS.
8. YOU MAY WONDER WHY WATER
IS TINT-FREE IN OUR
EVERYDAY LIVES?
THE REASON IS THAT RAYLEIGH
SCATTERING NEEDS LARGER
QUANTITIES TO BECOME EVIDENT.
THE MORE WATER IS ADDED, THE
BLUER IT WILL LOOK.
9. CURIOUSLY, AN OCEAN OF THE
SAME CHEMICAL COMPOSITION
AS ON EARTH, BUT LOCATED ON
SOME FAR-AWAY PLANET
CAN BE A COMPLETELY DIFFERENT
COLOUR, SUCH AS VIOLET.
10. WE (SADLY!) DO NOT HAVE
VIOLET OCEANS BECAUSE:
1. HUMAN EYES AREN’T GREAT
AT DETECTING VIOLET
2. IT FORMS ONLY A MINOR PART
OF OUR SUN’S SPECTRUM
3. IT CAN BE ABSORBED BY THE
OZONE LAYER
11. WHY IS THE OCEAN BLUE?
WELL, SOMETIMES
IT ISN’T!
