Iceland spar

[Richard Lundberg]

Interesting and humorous article on calcite and the Vikings and the history of optics.

http://www.lensrentals.com/blog/2013...covered-optics

[ajohnw]

It was used extensively to determine what is in rock samples and things like that fairly recently but calcite stopped being used not long after polarising films were introduced. Most of those are made by stretching a pieces of plastic film. Simply put that organises the molecules in a manner that turns the film into a polariser. When 2 are crossed very little light passes through so if a sample is placed between them some of the light is depolarised and passes though the other one. This for instance is a sugar crystal. A near keeper. Preparing samples is rather interesting. This one was left open and unsealed so that minor variations in dampness changed the colours. Doh and then some black spots appeared. The black cross is for making measurements.



This is by a master of taking shots like this. Preparation involves a lot of perseverance. He can also obtain a wide range of chemicals.

http://groups.yahoo.com/neo/groups/M...box/578285009#

John
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[Downrigger]

Interesting... I wouldn't have thought polarization would persist through cloud cover. This also reminds me of the use of natural light polarization for orientation by terrestrial (bees) and marine arthropods (lobsters, various plankton). It also reminds me, then, of the fact that though we see only a fairly narrow and unelaborated spectrum of radiation, it seems a pretty rich show and gives us a lot of pleasure.