The Earth Story

Yellow optical calcite being used to demonstrate double refraction - watch how the letters are split into 2 light rays that emerge at separate places from this crystal.

Double Refraction This is a crystal of the mineral calcite, a mineral with an extremely high birefringence. Birefringence is a property of how some transparent materials interact with light. A birefringent crystal like calcite has one direction where light moves rapidly through the crystal and another direction where light moves more slowly. When a beam of light enters a crystal of calcite, it splits into 2 separate rays, each vibrating (or polarized) in a different direction. When light moves into a crystal from the air, it refracts or bends, and the path it takes depends on how fast it moves. In other words, light enters a crystal of calcite and splits into 2 different beams. The light coming in from behind this crystal started as a single beam, but as it entered the calcite it split into 2 separate rays, each taking a different path through the crystal. When the light exits the crystal to head towards your eyes, each ray exits at a different point, so the image that you see behind the crystal is duplicated when the light comes out. -JBB Image credit: https://flic.kr/p/22SFK4v Read more: http://bit.ly/2P1NIWO

Viking Compass

Vikings were known for their prowess in sailing and navigating large stretches of open water 1000 years ago. But how did they accomplish these feats of navigation before the widespread availability of magnetic compasses in Europe? Icelandic sagas detailing aspects of Viking life may hold a few clues. Some sagas mention the use of rocks, named sunstones to find the location of the Sun to navigate by. The property that would have enabled this handy feat is called polarization. You can think of these polarized crystals as having fast lanes and slow lanes for light. Just like in a congested traffic jam where you would want to be in the fastest lane to get to where you are going, light will preferentially follow the “fast lane” in the crystal, causing light to be directed in a particular orientation in the crystal. Researchers suggest that Icelandic Spar would be the most likely choice for a sunstone, it is a rock that would be familiar to the Vikings comprised of calcite, a mineral that is birefringent, meaning there are two “fast lanes” that light can travel through. How would this be used to find the Sun’s position? Well, if the crystal was positioned so that light from the sun was able to pass through a calcite crystal’s “fast lanes” equally, it would be the only orientation of the crystal where both beams of light present in the crystal were equally as bright, enabling the user to find where in the sky the sun is, even on a cloudy day!

Researchers led by Guy Ropars from the University of Rennes in France tested this concept out using a piece of Iceland spar recovered from a sunken British ship and found that they were able to find the location of the Sun within 1% of its true location. While this study reveals that is possible to find the Sun’s location using a sunstone, a Viking sunstone has not been found on any recovered Viking ships. Because of this, researchers do not know for sure if Vikings used sunstones to navigate in the past, but it is a possibility that would have helped them navigate before the widespread use of magnetic compasses in Europe!

-AND

Image Credit: https://flic.kr/p/9xh4ML -Photo by Mike Beauregard

Sources: http://go.nature.com/2veNIvE http://bit.ly/1CWPlGA http://bit.ly/2tQmeMB_ _