The Purple Sky

Light, sight, and why things may not be the same colours they look.

Did you know that the sky looks blue? You probably did, specially if you’ve seen it. But just because it looks blue doesn’t mean it is blue — unless you’ve decided that that’s what “being blue” means. What’s more, it might only be blue to you.

Actually, the sky only looks blue if you’re down at the bottom of it. If you float up closer to the surface, you’ll find you can see right through it, out into the blackness of space.

So why does the sky look blue? That’s because what you’re actually seeing is light from the Sun coming onto your eyes. That light has to travel all the way through the air to reach you. And light can get distracted.

Light is basically rays of travelling energy — or, to put it another way, energy likes to travel light. But some light rays have more energy than others. Humans’ eyes detect these different energies, and report it to their brains as different “colours”.

The lowest-energy light-rays my eyes can see look “red”. This is followed by “orange”, “yellow”, “green”, “blue”, “indigo”, and finally “violet” which has a higher energy than all the rest.

Sometimes light lines up nicely in order of energy and people get to see a pattern in the sky called a “rainbow”.

Now, there are lots of tiny little things like dust particles floating around in the air. When light from the Sun comes in, it can’t travel straight because it keeps bumping into these things and changing direction. That’s why light eventually spreads across the whole sky, instead of coming straight to you and leaving everything else black.

If you shine a torch at night, you can usually see some of the particles in the ray of light that comes out. If that doesn’t work, try going to a mattress or curtain that hasn’t been cleaned for a while, bumping into it, and then looking again.

If it wasn’t for the air and the dust,the ray coming from your torch wouldn’t be so fuzzy. It would be as sharp as a laser.

Dust particles are very large compared to light, so they treat all light the same way. But there’s also the air itself — made out of very small molecules of gas. These molecules are small enough that they act differently for different kinds of light. Specifically, they scatter light with shorter wavelengths more than they do longer ones.

A wavelength is basically a measure of how a light-wave vibrates, or, to be more precise, how long those vibrations are.

Violet light has more energy than red, so it can vibrate faster and fit more waves into the same amount of space. That means it has a smaller wavelength than redder light, and gets scattered more.

So light from the violet end of the rainbow gets spread out much more than the ones near the red end. That’s why most of the light you see in the sky is indigo and violet.

Wait a minute — isn’t it supposed to be light-blue? No, it’s not light-blue.

The dragonfish is well suited to life under the ocean. It uses an invisible searchlight on the front of its head to find its prey. Invisible, that is, to most ocean creatures. But not for humans like you. To you, it would look red.

That’s because, deep down in the ocean, there is no red. To reach the depths, light has to travel through many miles of water, always getting weaker as it goes. Red light, with its lower energy, never manages to make it that far.

This helps the dragonfish, because it means that most of the creatures around don’t have eyes that can detect red light. Why bother to try looking at something that isn’t even there?

But of course, it is there. The dragonfish makes it. And that’s how it can see all but none can see it.

Not being able to see red is a very basic form of “metamerism”. To the deep-sea creatures, the colours “red” and “black” both seem like the same colour — which is to say, in this case, no colour at all.

Metamers are colours which are made up of different kinds of light, but are seen by eyes as being the same.

Human eyes have many colour-detecting cells called “cones”. There are three kinds of cone, each designed to detect a particular primary colour: red, green or blue. Of course, they don’t detect just those specific colours. Instead, they detect specific regions is the light spectrum.

A spectrum is basically what you get if you take the rainbow colours and lay them out in a line. Most human eyes have cones for light from red to yellow, from yellow to blue, and from blue to violet. But the colours they detect kind of overlap, so some lights can activate more than one kind of cone.

Now, cones don’t tell you the exact colours of the light. They kind of average it out. So a ray of red mixed with a ray of yellow will look the same as a ray of pure orange. Even though the second one is one, and the first is two kinds of light travelling together.

These combinations of light, that are different but look the same, are called “metamers”.

Metamers are different for different people. If you could see only black and white you’d find a lot more, with red looking the same as very dark blue and light yellow looking like pale pink. (If you can’t see things in only black and white, try an Instagram filter instead).

In the bluish-violet sky, the indigo light also activates a bit of the green cones, and the violet also activates a bit of the red cones. What’s more, the sky itself will have a bit of green and red light in it, though not quite so bright.

Red and green with blue makes white. But since the blue is stronger, there’s still some left over, and the sky looks like it’s a pale blue. There are the metamers at work again!

Some humans have cone cells which are slightly different from most, and detect slightly different colours. What looks green to most may look to them like a light peach (the inside of the peach, not the skin). Or maybe they’ll look at the sky and call it “purple”.

What actually happens is that their metamers are different. You might call these people “colour-blind” — but it may actually be the other way round!

Is it just about different cone cells? Not quite. The colours you see can also depend on the culture you grew up in. The Himba people of Namibia, for example, have the same word for blue and green— but they have many words for different types of green, which other people can’t make out so easily.

As for the sky, does it even have a colour? After all, it’s mostly thin air! The linguist Guy Deutscher decided to do an experiment on his daughter, Alma. Alma’s parents brought her up teach her the colours as usual, but they made sure never to mention the colour of the sky.

Finally, one day, her father pointed up to the empty sky and asked, “What colour is that?”.

At first, Alma was surprised at the question. It hadn’t occurred to her that the sky could have a colour. After a few days of thinking, she came up with the answer: white. It was only later that she first mentioned blue as a possible colour, and then it kept switching for a while before she finally settled down on blue.

That’s just one person, of course. But it makes one wonder: what’s the real reason the sky is blue? Maybe it’s just because someone told you so.

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An earlier version of this article was published in Sirius #244 30 Apr—13 May 2017 “The Purple Sky”.