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Author Topic: What causes iridescence on the backs of some beetles?  (Read 8732 times)

Offline John Chapman

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What causes iridescence on the backs of some beetles? I understand what causes the rainbow colours produced by films of oil on water and the walls of bubbles. And as far as I know these are both dependent on the oil or soap being transparent so that light is reflected by the surface on each side of the film, producing two wavebands of light which are out of phase and result in interference. But that canít happen with the opaque wing cases (the elytra) of a beetle. So what causes it? And is it the same thing that produces the pearly finish you find inside seashells?
 


 

Offline Chemistry4me

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What causes iridescence on the backs of some beetles?
« Reply #1 on: 16/03/2009 05:19:04 »
Like this?

 

Offline John Chapman

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What causes iridescence on the backs of some beetles?
« Reply #2 on: 16/03/2009 09:55:18 »
Excellent example. As you change your viewing angle the colours shift, just like it does with a soap bubble.
 

Offline LeeE

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What causes iridescence on the backs of some beetles?
« Reply #3 on: 16/03/2009 11:57:20 »
The iridescence you see on the backs of some beetles, and also bird feathers, camera lenses and some sunglasses, occurs for the same reasons that it does with oil or soap films.  However, although interference is a factor in the phenomenon, it is not really the interference that causes the colours but rather that the interference filters out other colours during reflection so that only light in a narrow band of colours is actually reflected, producing almost monochromatic light with a very pure colour.

The iridescence occurs due to the internal reflection of light within a transparent film or layer of material (or in photonic crystals, where their periodic nanostructure produces the effect of internal layers) where the thickness of the film or layer is of the order of the wavelength of the incident light.  Where the wavelength of the light matches or is a low multiple of the path length through the the film or layer it will re-emerge from the layer after a single reflection but where the wavelength is different to the path length it will be re-reflected within the layer until it either emerges after several internal reflections, if it's wavelength is a multiple of the internal reflection path length, or is absorbed by the material of the layer, or the material at the boundary of the layer.

Changing your viewpoint changes the path length through the layer and this means that a different wavelength or colour of light will seem to be reflected.
 

Offline BenV

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What causes iridescence on the backs of some beetles?
« Reply #4 on: 16/03/2009 13:34:54 »
Very well said.
 

Offline John Chapman

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What causes iridescence on the backs of some beetles?
« Reply #5 on: 16/03/2009 21:42:07 »
Hang on. That's a lot to get my head round.


Where the wavelength of the light matches or is a low multiple of the path length through the the film or layer it will re-emerge from the layer after a single reflection but where the wavelength is different to the path length it will be re-reflected within the layer


So it will only allow a specific colour to pass out. But why?




Changing your viewpoint changes the path length through the layer and this means that a different wavelength or colour of light will seem to be reflected.


So by changing the angle from which you view it the light will have to pass through a different thickness of the transparent surface to pass out in your direction. In other words the light that you see reflected is travelling more obliquely through the transparent layer to reach you if your viewing angle is more acute. I understand that bit (if, indeed, I have got it right). But I don't understand the bit about why it will only allow light to pass through that has it's wavelength the same or in multiples of the thickness of the transparent layer.
 
 

Offline LeeE

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What causes iridescence on the backs of some beetles?
« Reply #6 on: 18/03/2009 04:56:18 »
I thought I'd better have a quick read-up on this, to check my memory, and it turns out that it's due to a combination of phase and interference effects.

The thickness of the layer is the most important factor and still has to be of the order of the wavelength of the incident light.

When light enters the layer, some of it will be reflected directly from the front surface of the layer and some will enter the layer, but then be reflected from the rear surface of the layer.  Depending on the length of the path through the layer of the light that is reflected from the rear surface, it may then re-emerge through the front surface in phase with the light reflected from the surface, reinforcing that particular wavelength, or out of phase, in which case the light will be attenuated.

At least, I think that's the gist of it.
 

Offline John Chapman

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What causes iridescence on the backs of some beetles?
« Reply #7 on: 18/03/2009 09:43:59 »
Oh, so it's exactly the same as with a soap bubble? As described in the original question?
 

lyner

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What causes iridescence on the backs of some beetles?
« Reply #8 on: 22/03/2009 23:14:09 »
The difference between the wings, feathers etc. and a bubble is that the 'fabricated' version has several layers and the refractive indices of the layers are chosen (evolved with) for maximum effect. The mechanism is called an interference filter and can produce much more vibrant colours (more light reflected) than are possible with pigments. The same basic chemicals can give you any colour you want.
 

Offline JnA

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What causes iridescence on the backs of some beetles?
« Reply #9 on: 23/03/2009 00:55:52 »
wait is that the same as the butterfly wings?  they seem much more complex...
 

Variola

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What causes iridescence on the backs of some beetles?
« Reply #10 on: 23/03/2009 09:21:21 »
So what benefit does it have for the beetle??

If for example, it is to warn off predators, like birds, do birds see the same way we do??
If it was to warn off predators, then why isn't it brighter like some caterpillars, frogs and butterflies??
 

lyner

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What causes iridescence on the backs of some beetles?
« Reply #11 on: 23/03/2009 19:03:57 »
wait is that the same as the butterfly wings?  they seem much more complex...
Butterflies have scales on their wings, I believe - a more complex arrangement but the substance (chitin) and the interference principle is the same.

The advantages of colouration are very difficult to ascertain, sometimes. It's a balance between impressing a potential mate and dissuading a potential predator.
 

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What causes iridescence on the backs of some beetles?
« Reply #11 on: 23/03/2009 19:03:57 »

 

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