Why the sky is blue, why the sea is blue, and why almost nothing else is

IAir

Why is the sky blue?#

Sunlight arrives white: every colour at once, from violet near 400 nanometres to red near 700. The spectrum in the corner shows the light of the sky overhead.

Air throws blue light around#

Air molecules are about a thousand times smaller than a wave of light. They scatter short waves much more strongly than long ones, in inverse proportion to the fourth power of the wavelength.

That is how much more strongly blue light at 440 nm is scattered than red light at 680 nm. John William Strutt, later Lord Rayleigh, worked out the law in 1871.

Why isn’t the sky violet?#

Look anywhere away from the Sun and the light reaching you has been knocked off course on its way through the air, and most of it comes from the short end of the spectrum. Violet scatters even more, but sunlight carries less of it and the eye sees it poorly. The mix reads as blue.

Why are sunsets orange?#

As the Sun goes down, its light crosses more and more air. With the Sun on the horizon, the beam passes through about 35 times as much air as with the Sun straight overhead. Along the way almost all of its blue is scattered aside. What is left of the Sun is orange.

Why does the sky stay blue after sunset?#

Once the Sun is below the horizon, the light reaching the sky overhead has taken a long, grazing path high in the atmosphere, through the ozone layer, mostly 15 to 35 kilometres up. Ozone absorbs orange and yellow: near 600 nm an ozone molecule absorbs about 32 times as much as at 440 nm. In 1953 the physicist E. O. Hulburt showed that this is where the blue of the sky after sunset comes from. Without ozone, it would be yellowish.

The twilight sky for any place and evening: After sunset, the sky is blue for a different reason.

On Mars it runs the other way#

The Martian air is thin and full of fine dust, which turns the daytime sky butterscotch. At sunset the dust scatters blue light forward, close to the direction of the Sun, so the glow around the setting Sun is blue. NASA’s Curiosity rover photographed one in Gale crater in 2015.

The Martian sky on screen is painted, not calculated.

IIWater

Why is the sea blue?#

Now go under. Water does something air hardly does: it absorbs light, and it starts with red. The bonds inside each water molecule vibrate and soak up the longest waves. A glass is too thin to show it; a few metres of water are not.

The colours come from the absorption spectrum of pure water measured by Robin Pope and Edward Fry (1997). Depth: 0 m

Red goes first#

By about 7 metres, 99% of the red light at 700 nm has been absorbed. A red fish looks grey down here. Divers carry torches to see colours that daylight no longer brings.

Then orange, yellow, green#

In pure water, orange light at 600 nm is down to 1% by about 21 metres, yellow by about 51, green by about 80. The spectrum in the corner loses its colours from the right.

What colour is the deepest sea?#

What is left is blue, then violet. Pure water lets light near 420 nm through best: counting only what pure water absorbs, 1% of it would reach about 1,010 metres. Even the clearest natural water known, in the South Pacific, absorbs about twice as much there, so its light gives out much sooner, but its colour stays blue-violet. In most of the sea, plankton and particles take more light: according to NOAA, significant light ends at about 200 metres.

From here down, the screen dims as the real ocean does; its hue stays that of pure water.

Below 1,000 metres#

Below 1,000 metres no sunlight arrives at all. The dark is not empty: most sea animals that make light make blue or blue-green light, the colours that travel furthest through water.

The fish with a red lamp#

A few dragonfish break the rule. The stoplight loosejaw, Malacosteus, shines a red light that most deep-sea animals cannot see, and reads its reflections with a pigment derived from chlorophyll, which it probably gets from its food. A searchlight in a world that sees mostly blue.

IIILife

Why are so few animals blue?#

Look for blue in living things and you find mostly structures. Red, yellow and brown come from pigments. Blue pigments are so rare in animals that the few known cases are well studied. Most of the blue in animals is built, not dyed.

A wing that cancels colours#

The scales of a Morpho butterfly carry ridges with thin layers of chitin and air, a couple of hundred nanometres apart. Light reflected from each layer adds up for one band of wavelengths and cancels for the rest. The spacing decides which colour survives.

On screen: 8 chitin layers 70 nm thick; gaps of 215 nm reflect most strongly at 648 nm.

The same interference colours a soap bubble: The last colour of a soap bubble.

Change the gaps, change the colour#

Wet a Morpho wing with alcohol and it fills the air gaps. Light slows down in the gaps too, the stack reflects a longer wave, and the wing turns green. When the alcohol dries, the blue comes back. In the model on screen, ethanol moves the peak from 468 to 547 nm.

Crush the feather, lose the blue#

A blue jay’s feather works on the same principle, with a sponge of keratin and air. Grind it and the structure breaks; what remains looks brownish grey, the colour of the melanin underneath. On screen the layers are scrambled and the reflection spreads across every colour.

The sky in an eye#

Blue eyes contain no blue pigment. The front of the iris holds very little melanin, and its tissue scatters short wavelengths back out: the physics of the sky, at the scale of a few millimetres. The iris on screen is coloured with the noon sky from the first chapter.

IVWords

When did people start naming blue?#

Homer’s sea is oínops póntos, the “wine-dark sea”. In 1858 William Gladstone, later Britain’s prime minister, went through the Iliad and the Odyssey word by word and found no plain word for blue. The nearest, kyáneos, often just means dark.

Blue comes late#

In 1969 the anthropologists Brent Berlin and Paul Kay compared 98 languages, 20 of them tested with speakers, and proposed an order. Every language names dark and light. A third colour word is red. Then come green and yellow, and only then a separate word for blue. Many languages still use one word for green and blue: in the World Color Survey, 64 of 110 name typical green and typical blue alike.

Where each language draws the line: Where Blue Ends.

Two blues#

Languages cut blue in different places. Russian has two basic words: siniy for dark blue and goluboy for light blue. In a 2007 experiment, Russian speakers were quicker to tell two shades apart when one was siniy and the other goluboy. English speakers, who have one word, showed no such edge.

A green light called blue#

The Japanese word ao long covered green as well as blue. Green traffic lights in Japan are still called ao. From 1973, new signals used a bluer shade of green, closer to the word.

VStone

Where does blue pigment come from?#

If nature rarely makes blue, people had to. By about 2600 BC, Egyptian craftsmen were routinely heating sand, copper and lime to make Egyptian blue, generally considered the oldest synthetic pigment. Its earliest traces are about 5,200 years old.

Under red light it glows in the infrared, which is how conservators find its traces on ancient statues that look bare.

From beyond the sea#

Ultramarine came from one stone, lapis lazuli, mined in the mountains of Badakhshan in Afghanistan. Its name means “from beyond the sea”. In Renaissance Italy it could cost more than gold, and painters’ contracts named it for the robe of the Virgin.

An accident in Berlin#

Around 1706 in Berlin, the colour-maker Johann Jacob Diesbach was preparing a red pigment with potash contaminated by animal oil, and got a deep blue instead. Prussian blue was cheap, it travelled as far as Japan, and Hokusai printed The Great Wave with it around 1831. Cobalt blue arrived in 1802.

Gold made from soda#

In 1824 a French society offered a prize for artificial ultramarine. Jean-Baptiste Guimet made it in 1826 and collected the prize in 1828. The stone that cost more than gold became a factory product.

A blue in a sealed envelope#

In 1960 the artist Yves Klein deposited the formula for his International Klein Blue with the French patent office, in a sealed, dated envelope. It was not a patent. The secret was a binder that leaves ultramarine as matt and velvety as the loose powder.

The newest blue#

In 2009 a laboratory at Oregon State University, looking for materials for electronics, heated oxides of yttrium, indium and manganese and pulled out a vivid blue. Its discoverers called it the first new inorganic blue pigment in about two centuries.

The pigment colours on screen are close matches, not measurements.

VILight

Why was the blue LED so hard to make?#

Red light-emitting diodes glowed in 1962. Yellow and green followed. Blue would not come. The crystal it needs, gallium nitride, was close to impossible to grow well, and for three decades most laboratories gave up.

Three decades later#

Isamu Akasaki and Hiroshi Amano in Nagoya, and Shuji Nakamura at a small chemical company called Nichia, cracked it around 1990. Bright blue LEDs went on sale in 1993. The three shared the 2014 Nobel prize in physics.

White is blue in a yellow coat#

Most white LED light, in lamps and in screen backlights, is a blue LED under a yellow phosphor. The spectrum in the corner shows it: a sharp blue spike, then a broad hump. Much of the white light around you started as blue.

VIIDistance

Why is Earth a blue dot?#

On 14 February 1990, Voyager 1 was about six billion kilometres from home. At Carl Sagan’s urging, it turned its camera back toward the Sun and its planets.

A pale blue dot#

Earth came out as a single point of light, about 0.12 of a pixel, caught in a band of scattered sunlight.

Blue, for the reasons in this story#

Air throws blue light around. Water keeps blue the longest. From far enough away, that is all that is left of the place where everything has happened.