Polarized ice grains from shallow firn core
Here are small thin sections of firn ice showing grain structure through our polarization set-up. We took this picture with a simple hand held camera (non SLR) with the "Super-macro" setting.
From the explanation on the other page: As the light goes through the first polarized sheet, only polarized light of one orientation gets through. As it goes through the second polarizing sheet, only light that has that polarizing sheet goes through. If the two polarizing sheets are held in the same (parallel) orientation, light goes through. If you turn the upper sheet 90 degrees, no light should get through because light oriented one way by the first sheet isn't oriented to get through the second sheet. So by turning the second plastic sheet around, it goes dark. (can do same trick with two pairs of polarized sunglasses).
But ice grains can change light polarization as light bounces around the oriented crystals in the ice grains. A thick section of ice will let light through when it is sandwiched in between the two polarizing sheets. If the ice section is thin enough, the different orientations of the light coming through one set of grains can be seen as varying levels of brightness due to only a slight randomization of the polarization orientation. (If too thick, the light polarization is fully randomized).
Polarized ice grains from shallow firn core
Here are small thin sections of firn ice showing grain structure through our polarization set-up. We took this picture with a simple hand held camera (non SLR) with the "Super-macro" setting.
From the explanation on the other page: As the light goes through the first polarized sheet, only polarized light of one orientation gets through. As it goes through the second polarizing sheet, only light that has that polarizing sheet goes through. If the two polarizing sheets are held in the same (parallel) orientation, light goes through. If you turn the upper sheet 90 degrees, no light should get through because light oriented one way by the first sheet isn't oriented to get through the second sheet. So by turning the second plastic sheet around, it goes dark. (can do same trick with two pairs of polarized sunglasses).
But ice grains can change light polarization as light bounces around the oriented crystals in the ice grains. A thick section of ice will let light through when it is sandwiched in between the two polarizing sheets. If the ice section is thin enough, the different orientations of the light coming through one set of grains can be seen as varying levels of brightness due to only a slight randomization of the polarization orientation. (If too thick, the light polarization is fully randomized).