Zim
Today I'm excited to welcome Dr. Ellen Keister, a physicist and educator who bridges the worlds of light, optics, and visual science. She's the director of education for a research center that brings together experts from multiple disciplines to solve some of the hardest problems in imaging science. She has a background in laser and molecular physics and brings a unique perspective on how light behaves, both scientifically and artistically. In today's podcast, I want to talk a little bit more about polarization and polarizing filters.
Zim
Although I covered this way back in episode 19 with Dr. Rebecca Tileman at the University of San Diego, it's been so long, I thought I'd go over it again. Hey, Dr. Keister, welcome to the podcast.
Ellen
Hi, I'm really happy to be here, and you can call me Ellen.
Zim
Some of you may recognize the Keister name because I interviewed Ellen's sister, Dr. Jim Keister, for my podcast about what to expect when heading to Antarctica. So right off the bat, my first question is this, what the heck is polarized light?
Ellen
Polarization is a property of light, and the best way to understand it is if you think about a rope. So imagine you're holding onto the end of a rope and the other end of it is tied to something, and you're shaking the rope up and down, you'll get a series of waves that travel down the rope and back again. So we would consider that if you shake it up and down, we would call that vertical polarization. And if you shake it horizontally, we would call that horizontal polarization.
For a light wave, it's not actually a rope that's shaking, it's the electric and magnetic fields. The analogy of a rope works pretty well for that. That's what linear polarization is. If you are looking at information about a filter and see someone referring to linear polarization, linear polarization is either horizontal or vertical, just like a rope.
Zim
The light does that when it's coming from the sun or because it's bouncing off of an object?
Ellen
All light rays or light waves have a given polarization regardless of where they're coming from. Most light that you interact with in the world that's coming from the sun or that's coming from reflections or scattering off of objects is what we call randomly polarized, which means it's just as likely to be polarized horizontally or vertically or any direction in between.
Only certain interactions and certain materials, certain reflections, have a given polarization that's sort of distinct and not just randomly polarized. So all light has a polarization. It's just usually not very interesting.
Zim
One of the problems that we find in photography is when light reflects off shiny objects like water. My understanding is that when that light bounces back at us, it's polarized and it's polarized in a very predictable manner. Is that correct?
Ellen
Yes, that is correct. So when light bounces off or reflects off a shiny surface, whether it's metal or a window, glass, water, any of those things, it has a very specific polarization due to the way that the light waves interact with that surface.
Zim
And this is why we use a polarizing filter. That filter is there to cut that shine, is that correct?
Ellen
You can. It depends on what you're going for. You can cut the reflection off if artistically that's what you're trying to do, depending on the orientation of the filter. So if you orient the filter 90 degrees to the polarization that's being reflected off of the surface, you will eliminate that light that's coming from the surface or the reflection. If you rotate it to align with that polarization, it won't do anything. And then anything in between will give you something in between in terms of the reflection.
So if you want to see into the water and you don't want to see the reflection off the water, you would rotate it 90 degrees to the polarization reflecting off the water, and then you would see into the water.
This is also why a lot of sunglasses are polarized. They are typically polarized to cut out the glare, often from the surface of the hood of your car, or liquid water or other things on the surface of the road. So it's a safety thing. Or fishermen often have very specialized polarized lenses so that they can see into the water to see the fish.
Zim
Okay, so right now we're talking about reflections off of shiny surfaces. My understanding is when we point a polarizing filter up towards the sky, it also makes the sky more blue. What's happening there?
Ellen
The reason that the sky is polarized in certain directions depending on the angle of the sun is a lot more complicated to explain and has to do with the way that the light from the sun is scattering off of particles in the atmosphere. So if we didn't have an atmosphere, we would just be black because there's nothing for the sun to scatter off of. And the way that it scatters gives us a range of polarization across the span of the sky, depending on the angle of the sun or the time of day. So if you look through a polarizing filter or polarized sunglasses and rotate them, you will see different parts of the sky get darker or brighter, depending on the time of day and the orientation of the polarizing filter.
In physics, we talk about the degree of polarization. And what we mean by that is basically how non-random the polarization is. So if the light is 100% vertically polarized, we would say that is maximum degree of polarization. If it's completely random, that has zero degrees of polarization. It's unpolarized. So the polarization of the sky varies across the span of the sky, and the maximum effect you will have when you rotate a polarizing filter or sunglasses is 90 degrees from the location of the sun. When the sun is straight overhead, that's along the horizon. When the sun is on the horizon, it's along a 90 degree arc running from north to south.
Zim
Switching gears a little bit, can you tell me what's the difference between a linear and a circular polarizer?
Ellen
Linear and circular polarization are two different states that the light can have. It can be linearly polarized, circularly polarized, or anything in between, which we call elliptical polarization. Linear polarization is all of the oscillation in a single plane. If you imagine moving your hand in a circle while shaking the rope, you'll get something like circular polarization.
Zim
So when we say circular polarizer, we're not talking about the shape of my filter, right?
Ellen
No, we're not. All filters are circular, so that's understandable. The reason camera polarizing filters are called circular polarizers is that the light that reaches the lens or sensor is circularly polarized. Cameras rely on reflected light for autofocus and metering, and linear polarization can interfere with that. Circular polarizers work across camera systems without disrupting metering.
Zim
So in other words, a linear polarizer is basically going to prevent your camera from doing its job.
Ellen
Sometimes, yes. And rather than worrying about when that happens, it's easier to just use a circular polarizer.
Zim
My circular polarizer has two parts to it and it rotates. Why do I need to rotate it?
Ellen
Think of it like a picket fence. Depending on how the light is polarized, you rotate the filter so the fence aligns or blocks the light. This lets you tune the effect artistically.
Zim
Why are polarizing filters pretty much useless with a snowscape?
Ellen
Because snow scatters light in all directions. Scattered light is unpolarized, so rotating the filter doesn't change anything.
Zim
Is this why when I'm shooting a long river or big lake I can't remove all reflections evenly?
Ellen
Yes. The polarization changes across the surface based on angles of light and reflection, so you can only block some areas at once.
Zim
Is there a difference in polarization at sunrise and sunset versus midday?
Ellen
Yes. Maximum polarization changes position in the sky depending on the sun’s location.
Zim
When shooting wide angle lenses, I sometimes get strange dark patches in the sky. Is that because polarization varies across the scene?
Ellen
Exactly. A wide-angle lens captures areas of the sky with different polarization levels.
Zim
Will there ever be a polarizer that works evenly across the whole sky?
Ellen
Not really. You’d need multiple polarization orientations in one filter, which is very complicated.
Zim
Get on it, Ellen.
Ellen
Right after teleportation.
Zim
Fair enough. Do you have anything else we should know?
Ellen
No, I think that was a pretty comprehensive overview.
Zim
Thank you so much for joining me.
Ellen
Thank you for having me.