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  • Welcome to the new site. Here's a thread about the update where you can post your feedback, ask questions or spot those nasty bugs!

Right-handed circular polarization.

Jerome Marot

Well-known member
I am not really sure where to post this....


As the weather in Munich is abnormally hot (which worries me), I found out that I have an infestation of green rose chaffers (cetonia aurata) in the house. I find one every second day trying to get out through the window glass. This green coleoptera is not a problem as they live on dead plant parts. They are also fairly common. I suppose that a female laid her eggs in one of the plant pots that spent summer on my balcony and which I put inside for winter. When I find another one, I simply put him or her outside. They normally play dead when handled, so they are easy to move around... and photograph.

The green rose chaffer is called this way because of its colour, which is a metallic green:

DSC03329.jpg

(playing dead. it is not)

Now, what is amusing here is that this strange colour is a so-called structural colour. It is entirely made with physics and is the complex result of the interaction of light with nano-structures on the beetle skin. The resulting colour is left-handed circularly polarized. Really.

Now, if you are wondering what that means, remember that after Minolta introduced the first AF camera (1985), we had to exchange our linear polarizer filters for circular polarizing filters. That is not the shape of the filter (they have always been round), but a property of light. The beetle also does the circular polarization trick, of the left-handed kind.

If you remember your physics course about polarization, you know that the so-called "circular polarizers" we have for our camera input linear polarization and output circular polarization, so by using them in reverse we should be able to select for circular polarization. It is just that the manufacturer does not tell us whether it is the left or right-handed sort. So I had to test a few, till I found the right one (which is the right-handed sort). Observed through any kind of polarizer (including all of the "circular" ones in the direction they are meant to be used), the beetle is still coloured. But turn the righ polarizer around and the colour disappear:


DSC03328.jpg
 
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Asher Kelman

OPF Owner/Editor-in-Chief
So, do the insects themselves recognize each other by the polarization “color” or can they control the extent or constancy of polarization. It feasible that they pass a charge through a layer and it increases or decreases the effect.

Now why do we see this a green?

Asher
 

Jerome Marot

Well-known member
So, do the insects themselves recognize each other by the polarization “color” or can they control the extent or constancy of polarization. It feasible that they pass a charge through a layer and it increases or decreases the effect.
The colour of the insect never changes. It is created by nanostructures and not by pigments, but it is a fixed colour. There is no clear reason why this bug evolved to have such bright colours and it is unlikely to be used for the insects to recognise each other.

Now why do we see this a green?
Because the size of the structures is such that it reflects that particular wavelength. Other related coleoptera are brown, for example.
 

Doug Kerr

Well-known member
Hi, Jérôme,

I am not really sure where to post this....


As the weather in Munich is abnormally hot (which worries me), I found out that I have an infestation of green rose chaffers (cetonia aurata) in the house. I find one every second day trying to get out through the window glass. This green coleoptera is not a problem as they live on dead plant parts. They are also fairly common. I suppose that a female laid her eggs in one of the plant pots that spent summer on my balcony and which I put inside for winter. When I find another one, I simply put him or her outside. They normally play dead when handled, so they are easy to move around... and photograph. <snip>
Thank you for that very informative essay.

Regarding the need to use a circular polarizer in our SLR cameras, the reason for this is discussed in some detail in this article:


Best regards,

Doug
 
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