to Earth Science Australia...

to Time and Space Science...

Double Vision - Iceland Spar the most important substance ever

iceland spar

Iceland spar is probably the coolest, most important substance ever...
The Vikings probably used it to find America.
Iceland spar was a key to discovering what light actually was.
Every great name in optics from Huygens, to Newton, to Fresnel, and even Edwin Land (of Polaroid fame) used Iceland spar.
A shortage of Iceland spar in the late 1800s was considered an emergency by almost every industrial nation.
Iceland spar helped win World War II and can make things invisible.

Iceland spar is a particulaly pure form of the mineral calcite (calcium carbonate)
But it has two very interesting properties...
First --- light rays entering the crystal become polarized, split, and take two paths to exit the crystal – creating a double image of an object seen through the crystal.
The constant fog and mist in the North Atlantic often make navigation by stars or sun impossible.
The Vikings called Iceland spar a ‘sunstone’ because the polarizing effect can be used to find the direction of the sun even in dense fog and overcast conditions.
With simple pieces of Iceland spar, discovered that the atmosphere polarized sunlight, that light from a comet’s tail was polarized, moonlight was partially polarized, and certain chemicals in solution were also polarized

double image

Second --- Iceland spar is birefringence, meaning it refracts light into two separate images.
By aligning two pieces of spar polarising prisims could be made, from which photometers and spectrophotometers (instruments that measured light intensity and character) were developed
Polarized microscopes were used in biology and medicine, but also in geology to assess the authenticity of gems and crystals.
For photographers and astronomers, crossed polarizers were used to assess optical glass and lenses.
Flaws and defects in the glass which were invisible to visual inspection showed very clearly when examined by polarizers.
When the United States entered World War II, one of its first needs was a small, accurate gun sight for its warplanes and anti-aircraft guns.
Aerial gun sights at the time were large bulky affairs with prisms and lenses.

icelandic spar animation

Edwin Land invented the optical ring sight, which consisted of two sheets of polarizing film and two optical wave plates sandwiching a crystal of Iceland spar.
Optical calcite polarizers are used today as polarizers and beam-splitters in the laser and fiberoptics industries.
Researchers have found that using a pair of shaped calcite crystals in certain media, they can effectively make small objects disappear in visible light

Want to know more about ROCKS AND MINERALS and MINERAL OPTICS
credit to: Roger Cicala Dec 12, 2013