One Flash of Light, One Vision: Carrots, Colour and Chemistry

“White” light is made up of all the colours of the rainbow.

Sometimes you have one of those weeks when the universe seems to be determined to yell at you about a certain thing. That’s happened to me this week, and the shouting has been all about light and vision (earworm, anyone?).

I started the week writing about conjugated molecules and UV spectrometry for one project, was asked a couple of days ago if I’d support a piece of work on indicators for the RSC Twitter Poster Conference that’s happening from 2-3rd March, and then practically fell over a tweet by Dr Adam Rutherford about bacteria that photosynthesise from infrared light in a hydrothermal vent*.

Oh well, who am I to fight the universe?

Light is awesome. The fact that we can detect it is even awesome-er. The fact that we’ve evolved brains clever enough built all sorts of machines to measure other kinds of light that our puny human eyes cannot detect is, frankly, astonishing.

The electromagnetic spectrum covers all the different kinds of light. (Image source)

Let’s start with some basics. You probably met the electromagnetic (EM) spectrum at some point in school. Possibly a particularly enthusiastic physics teacher encouraged you to come up with some sort of mnemonic to help you remember it. Personally I like Rich Men In Vegas Use eXpensive Gadgets, but maybe that’s just me.

The relevant thing here is that the EM spectrum covers all the different wavelengths of light. Visible light, the stuff that’s, well, visible (to our eyes), runs from about 400 to 700 nanometres.

A colour wheel: when light is absorbed, we see the colour opposite the absorbed wavelengths. (Image source)

Now, we need another bit of basic physics (and biology): we see light when it enters our eyes and strikes our retinas. We see colours when only certain wavelengths of light make it into our eyes.

So-called “white” light is made up of all the colours of the rainbow. Take one or more of those colours away, and we see what’s left.

For example, if something looks red, it means that red light made it to our eyes, which in turn means that, somewhere along the way, blue and green were filtered out.

(Before I go any further, there are actually several causes of colour, but I’m about to focus on one in particular. If you really want to know more, there’s this book, although it is a tad expensive…)

Back to chemistry. Certain substances absorb coloured light. We know them as pigments. Carrots are orange, for example, largely because they contain a pigment called beta-carotene (or β-carotene). This stuff appears, to our eyes, as red-orange, and the reason for that is that it absorbs green-blue light, the wavelengths around 400-500 nm.

β-Carotene is a long molecule with lots of C=C double bonds. (Image source.)

Why does it absorb light at all? Well, β-carotene is a really long molecule, with lots of C=C double bonds. These bonds form what’s called a conjugated system. Without getting into the complexities of molecular orbital theory, that means the double bonds alternate along the chain, and they basically overlap and… smoosh into one long thing. (Look, as the saying goes, “all models are wrong, but some are useful,” – it’ll do for now.)

When molecules with conjugated systems are exposed to electromagnetic light, they absorb it. Specifically, they absorb in the ultraviolet region – the wavelengths between about 200 and 400 nanometres. Here’s the thing, though, those wavelengths are right next to the violet end of the visible spectrum – that’s why it’s called ultraviolet after all.

Molecules with really long conjugated systems start to absorb in the coloured light region, as well. And because they’re absorbing violet and blue, possibly a smidge of green, they look… yup! Orangey, drifting into red.

So now you know why carrots are orange. Most brightly coloured fruit, of course, is that way to attract animals and birds to eat it, and thus spread its seeds. As fruit ripens, it usually changes colour, making it stand out better against green foliage and easier to find. This is the link with indicators that I mentioned at the start: many fruits contain anthocyanin pigments, and these often have purple-red colours in neutral-acidic environments, and yellow-green at the more alkaline end. In other words, the colour change is quite literally an indicator of ripeness.

But the bit of the carrot that we usually eat is underground, right? Not particularly easy to spot, and they don’t contain seeds anyway. Why are carrots bright orange?

Modern carrots are mostly orange, but purple and yellow varieties also exist.

Well, they weren’t. The edible roots of wild plants almost certainly started out as white or cream-coloured, as you might expect for something growing underground, but the carrots which were first domesticated and farmed by humans in around 900 CE were, most probably, purple and yellow.

As carrot cultivation became popular, orange roots began to appear in Spain and Germany in the 15th/16th centuries. Very orange carrots, with high levels of β-carotene, appeared from the 16th/17th centuries and were probably first cultivated in the Netherlands. Some have theorised that they were particularly selected for to honour William of Orange, but the evidence for this seems to be a bit slight. Either way, most modern European carrots do descend from a variety that was originally grown in the Dutch town of Hoorn.

In other words, brightly-coloured carrots are a mutation which human plant breeders selected for, probably largely for appearances.

But wait! There was an advantage for humans, too – even if we didn’t realise it straight away. β-carotene (which, by the way, has the E number E160a – many natural substances have E numbers, they’re nothing to be frightened of) is broken up in our intestines to form vitamin A.

Vitamin A is essential for good eye health.

Vitamin A, like most vitamins, is actually a group of compounds, but the important thing is that it’s essential for growth, a healthy immune system and – this is the really clever bit – good vision.

We knew that. Carrots help you see in the dark, right?

Hah. Well. The idea that carrot consumption actually improves eyesight seems to be the result of a World War II propaganda campaign. During the Blitz, the Royal Air Force had (at that time) new, secret radar technology. They didn’t want anyone to know that, of course, so they spread the rumour that British pilots could see exceptionally well in the dark because they ate a lot of carrots, when the truth was that those pilots were actually using radar.

But! It’s not all a lie – there is some truth to it! Our retinas, at the back of our eyes, have two types of light-sensitive cells. Cone cells help us distinguish colours, while rod cells help us detect light in general.

In those rod cells, a molecule called 11-cis-retinal is converted into another molecule called rhodopsin. This is really light-sensitive. When it’s exposed to light it photobleaches (stops being able to fluoresce), but then regenerates. This process takes about thirty minutes, and is a large part of the reason it takes a while for your eyes to “get used to the dark.”

Guess where 11-cis-retinal comes from? Yep! From vitamin A. Which is why one of the symptoms of vitamin A deficiency is night blindness. So although eating loads of carrots won’t give you super-powered night vision, it does help to maintain vision in low light.

Our brain interprets electrical signals as vision.

How do these molecules actually help us to see? Well, when rhodopsin is exposed to light, the molecule changes, which ultimately results in an electrical signal being transmitted along the optic nerve to the brain, which interprets it as vision!

In summary, not only is colour all about molecules, but our whole visual system depends on some clever chemistry. I told you chemistry was cool!

Just gimme fried chicken 😉


*Ah. I sort of ran out of space for the weird hydrothermal bacteria thing. At least one of the relevant molecules seems to be another carotenoid, probably chlorobactene. The really freaking amazing thing is that there seems to be an absorption at 775 nm, which is beyond red visible light and into the infrared region of the EM spectrum. Maybe more on this another day…


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Blue skies and copper demons: a story of mysterious purple crystals

Mystery purple crystals (posted with permission of Caroline Hedge, @CM_Hedge)

Today, a little story about some mysterious, purple crystals. On Tuesday, Twitter user Caroline Hedge posted this photo with the question: “What the %#&$ is lab putting down the drain to cause this?”

The post spawned lots of responses, some more serious than others. One of the sensible ones came from Roland Roesler, who thought that the pipe had corroded from the outside, suggesting that a leaky connection at the top right had allowed sewage to drip down the right-hand side of the copper pipe and drip from the bottom, which explained why the left-hand half of the pipe appeared unscathed.

I agreed. The pipe is clearly made of copper, and blue colours are characteristic of hydrated copper salts. Inside the pipe, the flow of water would wash any solution anyway before corrosion could occur, but on the outside, drips could sit on the surface for long periods of time. There’d be plenty of time for even a slow reaction to occur, and then for water to slowly evaporate, allowing the growth of spectacular crystals.

Hydrated copper(II) sulfate crystals are bright blue. (Image from Wikimedia Commons)

But what exactly where they? There were several theories, but for me the interesting thing was the colour. Hydrated copper(II) sulfate crystals are bright blue. The colour arises due to an effect called d orbital splitting, which is a tad complicated but, in short, means that complex absorbs light from the red end of the visible light spectrum, allowing all the other colours of light to pass through. As a result, our eyes “see” blue.

But these crystals, assuming it’s not a photographic effect, had a purplish hue. At least, some of them do. So… not copper sulfate, or not entirely copper sulfate (given the situation, a mixture seemed entirely likely). Which begs the question, which copper complex produces a purple colour?

A little bit of Googling and I was pretty sure I’d identified it: copper azurite, Cu₃(CO₃)₂(OH)₂. This fit for two reasons: firstly, it’s a mineral that could (does) readily form in the presence of water and air (which, of course, contains carbon dioxide), and secondly it’s exactly the right colour.

Many will recognise the word “azure” as being associated with the deep, rich blue of a summer sky, and in fact the English name of this mineral comes from the same word-root: the Persian lazhward, a place known for its deposits of another deep-blue stone, lapis lazuli (meaning “stone of azure”).

Blue-purple copper azurite and green malachite (image from Wikipedia)

Azurite is often found with malachite, the better-known green copper mineral that we recognise from copper roofs and statues. Malachite is sometimes simplistically described as copper carbonate, implying CuCO₃, but in truth it’s Cu₂CO₃(OH)₂ pure copper(II) carbonate doesn’t form in nature.

You can see malachite co-existing with azurite in the photo on the right. The azurite will, over time, tend to morph into malachite when the level of carbon dioxide in the air is relatively low, as in ‘normal’ air—which explains why we don’t usually see purple ‘copper’ roofs—but the carbon dioxide levels were probably higher in that cupboard. There was almost certainly acidic sewage reacting with carbonate, combined with a lack of ventilation, so it makes sense that we might see more azurite.

Azurite has an interesting history as a pigment. Historically blue colours were rare and expensive—associated with royalty and divinity—which is one reason why the Virgin Mary was often depicted wearing blue in paintings. Azurite was used to make blue pigments, but (as I mentioned above) it’s unstable, tending to turn greenish over time, or black if heated. Ultramarine blue (made from lapis lazuli) is more stable, particularly when heated, but it was even more expensive. A lot of blue pigments in medieval paintings have been misidentified as coming from lapis lazuli, when in fact they were azurite—a more common mineral in Europe at the time.

There’s a fun piece of etymology here, too. Copper, of course, has been valuable metal since, well, the Bronze Age. The presence of purple azurite and green malachite are surface indicators of copper sulfide ores, useful for smelting. This lead to the name of the element nickel, because an ore of nickel weathers to produce a green mineral that looks a little like malachite. And this, in turn, lead to attempts to smelt it in the belief that it was copper ore. But, since it wasn’t, the attempts to produce copper failed (a much higher smelting temperature is needed to produce nickel).

The mineral nickeline can resemble malachite, and was dubbed kupfernickel in Germany, literally “copper demon”

As a result, the mineral, nickeline, was dubbed kupfernickel in Germany, literally “copper demon”. When the Swedish alchemist Baron Axel Fredrik Cronstedt succeeded, in 1751, in smelting kupfernickel to produce a previously unknown silvery-white, iron-like metal he named it after the nickel part of kupfernickel.

And this is how we go from a corroded pipe to sky-blue colours to medieval paintings to copper demons to nickel. But what happened to the pipe in the original tweet? Well, in an update, Caroline Hedge told us that it had been removed and disposed of, and so we’ll never be completely sure what the pretty crystals were, but they certainly lead to an interestingly twisty-turny chemistry story.


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