Rainbow colours in a spider web caused by diffraction of light – from Earth Science Picture of the Day
Please indulge me while I share some shiny things with you.
I always get like this in Spring, when the sun grows stronger and nature has a certain sheen to it.
I think it’s a woman thing, a gatherer trait inherited from our Stone Age forebears, who must have spent their whole lives looking for bright, shiny berries to pick and eat.
And in many species it’s the male who displays and the female who appreciates it. Apparently not in humans, but diamonds are still a girl’s best friend. So here I am exploring both sparklies and more subtle lustres – as I look at the words iridescence, opalescence and pearlescence.
Diamonds are a girl’s best friend? This is a classic round “brilliant cut” example
First iridescence. The word comes from Iris, the name of the Greek goddess of the rainbow, and it means coloured like the rainbow or glittering with rainbow colours.
Iris the messenger and goddess of the rainbow, pictured with Morpheus (naughty bits cropped off by me) in this oil painting by Pierre-Narcisse Guerin at the Hermitage in St Petersburg
You can read more about Iris here. She had a twin sister Arke, who became a messenger for the Titans, so they were messengers for enemy sides, Iris working for the Olympian gods, long before mankind existed. Confusingly Arke was the one with rainbow-coloured wings, while Iris’s were golden. Although not in the painting above…
As for the flower “iris”, this is named for the fact that it comes in many different colours…
A rainbow of irises in this fabric design by Yolanda V Fundora …
I love rainbows and somewhere I have a triangular glass prism to create my own rainbows. When light goes from one medium (air) to another (glass or water) it is slowed down and this leads it to be refracted.
With a prism, you get the rainbow because the white light entering the prism is actually made up of a combination of lots of colours of light, each with a different wavelength. These are differently refracted and the distinctive shape of the prism means the colours split apart by the time they emerge on the other side. We call them red, orange, yellow, green, blue, indigo and violet (with the handy mnemonic “Richard Of York Gave Battle In Vain”).
Prism produces a rainbow by refraction of light…
But the rainbow colours in iridescence are not fixed but change as you move the angle you are looking from. It can be caused by thin-film interference or diffraction.
Thin-film interference is what you get in soap bubbles and slicks of petrol (gasoline) on the road. I can’t begin to explain the mathematical equations behind it, but see that link if you really want to know!
Iridescent soap bubble…
I think that is also the effect in this beautiful bismuth crystal…
The chemical element bismuth as a synthetic crystal. The iridescent surface is a very thin layer of oxidation. The cube is also very pure bismuth
Then there is iridescence caused by diffraction. Examples of this are CDs and DVDs and the halo around the moon caused by ice particles. In the CDs the tracks are so close together that the rays of light bouncing off them interfere with each other and causes the rainbow effect.
Iridescent compact discs…
When I was growing up we didn’t have a fridge or cellar, so I was always wary of food going off. I have never understood the perfectly OK iridescence you get on ham or bacon, or cooked beef (not that we could afford much beef in those days), but apparently that is also caused by diffraction of light. Interestingly the Daily Mail did a feature on it a year ago. See here.
Rainbow beef…
Structural colouration in nature seems to use one of the above effects (thin films or diffraction gratings), but Wikipedia gives it a separate section here. As it says: “Colours are produced when a material is scored with fine parallel lines, formed of one or more parallel thin layers, or otherwise composed of microstructures on the scale of the colour’s wavelength”.
Peacock feathers…
Peacock feathers contain brown pigment but the fine microstructures of the feathers produce a rainbow effect.
Iridescence on the breast of a feral pigeon in my garden this week…
Blue morpho butterfly – more iridescent in real life…
It’s sad to think so many of these wonderful morpho butterflies of the tropics were caught to be pinned and framed or even for their wings to be used as decoration in trinkets and pictures. Beetles have also suffered…
A beautiful iridescent beetle (unsure about the exact species)…
The epicuticle or outer surface of an iridescent beetle is made of many stacks of slanting, plate-like layers, orientated in different directions. These layers bend and then reflect the incoming light in the same way as the ridges of the wing scales of iridescent butterflies.
Like butterfly wings, beetle wing cases have long been used as decoration – as recently as 2012 Charlize Theron wore a dress embellished with beetle wings for her role as the wicked queen in the movie Snow White and the Huntsman…
Beetle-wing dress worn by Charlize Theron…
Thankfully I think these earrings do not use real beetle wings…
Ruthenium-plated Swarovski crystal scarab beetle clip earrings…
Rainbow-black jewellery and beads are often called “scarabé” after the iridescent scarab dung beetles so beloved of the Egyptians.
Vintage scarabé beads
Scarab art…
I’m not sure if it’s another name for EXACTLY the same thing, but you also find “carnival” beads…
Carnival bead necklace
And there are also vases and bowls made of carnival glass…
Westmoreland carnival glass – part of a range using the Maple Leaf pattern
So I would happily use the word iridescence for rainbow colours on transparent or black backgrounds. But when the colours are milky white? I would call that opalescence. Naturally this word comes from the appearance of opal, a form of amorphous silica, usually milky white with a play of colour. The word opal is an old one, Latin opalus, Greek opallios, perhaps from the Sanskrit upala, meaning a gem.
Wondrous “rough” opal from Coober Pedy mine in South Australia…
Opal polished into a smooth cabochon shape…
This is where my definitions get confused, as it seems not all opals are milky white…
Opals of the world illustrated on the Opal Auctions website…
This is a particularly magnificent one that is almost black, so there goes my neat demarcation between opalescence and iridescence…
Koroit opal from Queensland…
Some sorts of milky, rainbow-coloured glass are also called opalescent…
My own very cheap opalescent glass perfume bottle…
An opalescent glass Moissac vase by the famous René Lalique – possibly worth £2,000 or £3,000…
Here’s a lovely digital artwork illustrating opalescence…
Opalescent II by psion005 on DeviantArt…
But where do we stand with pearlescence? Does this word even exist apart from in cars with shiny paint-jobs?
It seems to be a late 20th century word and it doesn’t even appear in my Chambers Twentieth Century Dictionary of 1972 as the other two do. Online definitions I have seen make it pretty well a synonym for iridescence and opalescence.
Rainbows in mother-of-pearl – oyster shell from Mopelia Atoll, Society Islands, French Polynesia
Personally I see more room for it as a sort of lustre like a pearl, which doesn’t actually have much rainbow colouring to it, unlike mother of pearl, also known as nacre.
To my mind these “pearl abalone” shells are indeed pearlescent, with no trace of rainbow colours…
And this would fit in with the way it describes car paintwork – you can have flat colours, or metallic or pearlescent…
TVR Tuscan with pearlescent paintwork…
Except Wikipedia says: Nacre is an organic-inorganic composite material produced by some molluscs as an inner shell layer; it is also what makes up the outer coating of pearls. It is strong, resilient, and IRIDESCENT.
A polished Paua abalone shell – pearlescent, opalescent or iridescent?
I have a pendant made of abalone shell…
Dammit I’ve come full circle. I have to stick to my guns and insist that iridescent is rainbow coloured and opalescent is rainbow-coloured with milkiness. But pearlescent just doesn’t exist…
My plastic pearls…
We’ll just have to call pearly pearly…
Pearly kings and queens of London…
Read about pearly kings and queens (London’s ancient street traders or “costermongers”), who decorate their clothes with mother-of-pearl buttons, here…
The Squirrelbasket 
What an interesting post and so well researched. The beetle is amazing. It doesn’t look real. If it was 1st April I’d think you were having us on. Why does a beetle need to be so beautiful? He’d make a nice brooch. My grandmother used to have a butterfly’s wing brooch. Even as a child I thought it was a bit obscene decorating yourself with bits of animal.
I suppose it’s beautiful to attract a mate…
There’s something worse than using bits of animals in jewellery – do you remember when there was a craze for having a LIVE beetle tethered on a chain, walking over your lapel?
Or did I imagine that?
Best wishes 🙂
I never heard about that one. What an awful thought. It would poo all over your lapel and what would you feed it on? It all sounds a bit messy to me apart from being in the worst possible taste .Maybe you dreamt it.
Looks like I didn’t dream it!
See http://en.wikipedia.org/wiki/Live_insect_jewelry
and little video here… http://www.huffingtonpost.com/2012/03/27/maquech-broach-beetle-jewelry_n_1383319.html
Well,having a giant hissing cockroach stuck on your lapel is the stuff of nightmares.
not only Rainbow colours in a spider web your post also…….stunned this post and scroll down and down ……….so exciting…….great blog no words…………….creativity……..
Thank you!
Best wishes 🙂
Chloris has beaten me to it – I loved the beetle too. When I was a child I had a pendant which included those butterfly wings. Interesting post full of fascinating and beautiful things.
Thank you, Elaine, I did keep on going oooooh! and aaaahhh! as I was sourcing the images!
Best wishes 🙂
Do starlings count? The number of colours in their apparently dark brown feathers is extraordinary when caught in the right light. (Can’t say I’m tempted to eat your beef.
Did you see my post Starlings – ruffians in star-spangled jackets?
By the way, I must admit it’s not MY beef! If you click on it you go to the Daily Mail article…
Cheers 🙂
What and unusual post. ! So colorful bringing out nature itself Wonderful pictures.
Thank you. Very few of them my own, of course. But if people click on the images they will go to the sources 🙂
Great idea for a post.
Butterfly colours and irridescence are produced by several mechanisms, sometimes in combination, including both diffraction and thin-film interference and use both layers of cells and lattices. The study of how (and why) these structures evolved is an interesting branch of science in itself.
Of course, colour is a subjective construct – what one person perceives may be different to another, and what we typically see as a species may be remarkably different to other animals.
Here are some thoughts to end: What colour is the butterfly’s wing in the dark? Can it’s iridescence affect the famous ‘butterfly wing effect’ (part of chaos theory)? Why is an abalone shell lustrous and pearly only on the inside where no one can see it?
Thanks for your kind endorsement!
As you say, the iridescence of butterfly and bird wings is caused by quite complicated processes and I leave readers to investigate the details for themselves, as my posts are long enough as it is. I am OK with most elements of Optics, until it starts using equations…
That’s very philosophical, about us all seeing things differently. Not sure quite how we ever get agreement on anything, in terms of what to call things. We do all live inside our own heads. In fact do you really exist?
I would think that in the dark a butterfly’s wing is black, unless you mean something along the lines of is there a noise in the forest if a tree falls and no one is there to hear it…
I am familiar with chaos theory. Not sure if the iridescence is going to make much difference, except, as the poet Francis Thompson says:
“All things by immortal power,/ Near or far,/ Hiddenly/ To each other linkèd are,/ That thou canst not stir a flower/ Without troubling of a star…”
As for the abalone shell – I suppose the inside is iridescent only accidentally, not for any purpose. And nature possibly doesn’t care if we wonder at it or not…
Lots to think about!
Best wishes 🙂
Intriguing post! Never thought so thoroughly about shiny things before, even though I’m a huge fan of gemstones.
Thank you. I’m sure there is lots more that could be said. There are many gemstones I may write about one day…
Best wishes 🙂
Iridescence is a fascinating phenomenon, and I enjoyed your exploration of it. The Iris fabric is gorgeous.
Glad you like the textile. I gathered many more pictures of “rainbow black” before I began the post but ended up leaving them out. Glad I put the irises in for you, though…
Best wishes 🙂
A wonderful post of shiny things. For me examples from nature creates the best examples. Sarah x
Indeed.
I particularly like the abalones – and of course there are similar, smaller, “ormers” in the Channel Islands. I don’t think they live as far north as Dorset, but I expect they can be bought in those wonderful little seaside souvenir shops in Britain. In fact I think I have one somewhere!
Best wishes 🙂
Fabulous post. I love how you do this Pat. It must be time-consuming … thank you! Shaz
Well, it’s my hobby! Doesn’t take as long as you might think, though, as I do write words and process pictures in my day job, too, so am quite quick…
Thanks for your kind words.
Best wishes 🙂
paging Dr Roy G Biv – is another version which is now stuck in my head.
[…] Koroit Opal (Australia, South West Queensland ) |via| […]
Some lovely things here, too:
http://wisance.com/15-rarest-of-rare-awesome-things-found-on-earth-ever/