2015. január 23., péntek

Reversal of handedness of circularly polarized light by mirror

A demonstration – simple, cheap & instructive 

For this demonstration you need
  • a small pocket mirror (preferably rectangular)
  • a pair of 3D movie glasses (circularly polarized) 
  • a shiny green rose chafer (dead or alive :) 
The rose chafer (Cetonia aurata) is a very common beetle in Hungary where I live. It comes in different shades of shiny green but natural black also occurs.

Watch the black one to the left. For some reason it lacks the helical structure of chitin molecules that is needed to produce the shiny green color of its neighbor to the right. You will need a green one for the demonstration. I am showing you the black one only for comparison. That's how the green one would look if, somehow, you'd block the circularly polarized light that carries the green color to your eyes.

As you can learn from this Wikipedia article, the color of Cetonia aurata comes from left-handed circularly polarized light. You can easily check this using a circularly polarized filter. That's why you need the 3D movie glasses which have both types of filter. Mine is a masterImage make, whose left filter is left polarized and right filter is right polarized. Easy to remember, see? This video shows you the glasses in action. Make sure that the English SUBTITLES are turned ON!



And here comes the demonstration in three acts:


Act 1 – w/o filter

Without any filter, the beetles look the same on both sides of the mirror, which is not surprising, of course.

Act 2 – right filter to absorb left-handed light that makes Cetonia aurata colorful

This picture was taken through the right filter that absorbs all left-handed light. So if you see any color it must be associated with right-handed light. The beetles in front of the mirror lost their color as expected. Their mirror images, however, are colorful. At this point there can be two explanations of this:
(1) the handedness of light has been reversed by reflection;
(2) the reflected light has been depolarized in the mirror.

Act 3 – left filter to absorb right-handed light

This picture was taken through the left filter that absorbs all right-handed light. So if you see any color it must be associated with left-handed light. The beetles in front of the mirror kept their color as expected. Their mirror images, however, are black like the "handicapped" one in the first figure. There is only one explanations of this: 
the handedness of light has been reversed by reflection

Note that you can easily perform this demonstration as a field experiment. I only used these dead animals because I had them and it was already winter in Hungary when I came to the idea of doing it. Yes, and being dead they were a lot more patient while I was taking the photographs than those you find in the field busily eating away the petals of flowers and mating at the same time.

Note also that I found those beetles already dead. The headless one to the left is from my compost heap – probably a female that died after having laid her eggs in the compost. The 2D specimen is from the street – it was lying flat on the sidewalk. As you see, its abdomen is of different color than its back but obviously of left-handed light too.

Related links

Thumbnail of the GIF uploaded to Wikimedia


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