Zim: Today I'm bringing back Ellen Keister. She's a physicist at Colorado University, and she's here to answer some questions about polarization and light again. Welcome back, Ellen.
Ellen: Thanks for having me.
Zim: Today's question comes from Lori of Ottawa, Canada. Lori wants to know, because she's heading to Antarctica at the South Pole, the sun circles the horizon for months. How does the unique solar geometry reshape the polarization pattern of skylight compared with mid-latitudes?
Ellen: The polarization of the sky depends on the time of day as well as where you are on Earth, the latitude. So you will get a different polarization pattern in the sky depending on the angle of the sun, and because the Earth is tilted, that is more significant at the poles.
Zim: The next question, Lori asks, blue ice areas look "clean." Is that a true reduction in multiple scattering or a coherent backscatter or polarization effect that we could quantify?
Ellen: The white areas of ice, like the ice from your refrigerator, are due to air bubbles that are trapped in the ice. It’s similar to why clouds look white—they have large particles that scatter light. Glacial ice, on the other hand, is very dense and doesn’t have as many air bubbles, which is why it looks cleaner. The blue color of glacial ice comes from the fact that as light travels through it, the red end of the visible spectrum is absorbed, and the blue light is what reaches your eyes.
Zim: A number of years ago, I was photographing glaciers in Argentina, and they looked blue. A gentleman saw my image and said, "Oh, you had an overcast day, didn’t you?" I said, "Yeah, how did you know?" He explained that if it’s overcast, glaciers look blue, but in sunny conditions, glaciers look green. Why is that?
Ellen: When it’s overcast, you get very diffuse light from the sun, not direct sunlight reflecting off the surface, so it looks blue. When it’s sunny, you also get reflected light, like off a mirror or a lake. The combination of the blue of the glacial ice and the reflected sunlight gives a greenish tone.
Zim: So when I color-correct my images in Lightroom, I shouldn’t try to take out blue or green. The color difference is real—it’s not a mistake.
Ellen: Exactly.
Zim: The next question Lori has is, brine channels in sea ice create strong optical contrast. Can polarization spectroscopy map brine connectivity and predict melt point onset?
Ellen: This isn’t my main area, but I can speculate. Brine channels have higher salt content, so they have a different structure than the surrounding ice or water. You could use spectroscopy, if calibrated correctly, to measure the salt content and combine that with data on how salt affects melting points to predict when certain areas of ice might start melting.
Zim: The next question, do penguins or other Antarctic animals exploit polarized light—for glare reduction on snow or sea—and could that inform anti-glare design for humans?
Ellen: I’ve never heard of animals using polarization specifically for glare reduction. Some insects and birds have eyes sensitive to polarization, but typically they use it for navigation, sensing differences in polarization across the sky.
Zim: Also, as we discussed in previous episodes, polarization and polarizing filters don’t have much impact on snow. So even if animals were sensitive to polarization, you couldn’t actually cut snow glare with a filter.
Zim: Now I have one last question for you, because we’re headed to Antarctica. Earlier, you mentioned that at the poles, the light is at a very different angle than at the equator. We won’t get true nighttime, just dusk. What can I expect from the sky? Will there be unusual or different colors compared with the Northeast or continental U.S.?
Ellen: I wouldn’t expect different colors, but you might get an extended “golden hour” effect. Because it doesn’t get fully dark, the light immediately after dawn or just before sunset can last much longer.
Zim: Thank you so much for joining me today.
Ellen: Thanks for having me.
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