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Polarization gives spies an easy leg up

Posted: Friday, May 28, 2010

While out on a mission to survey for birds, my friend and I spent all day scanning the ocean from a skiff. Even on cloudy days, the glare sometimes made it difficult to distinguish the shapes of our feathered suspects. With polarized sunglasses, however, the glare seemed to vanish, revealing the silhouettes of marbled murrelets or pigeon guillemots.

Courtesy Of Beth Peluso
Courtesy Of Beth Peluso

How do the sunglasses work? The human eye can't distinguish between polarized light and non-polarized light. We experience it simply as glare. Sunlight is mostly scattered by the atmosphere, with light waves moving in all directions. The shortest waves at the blue end of the spectrum are scattered the most, giving the sky its blue color. In addition to the direction they travel, light waves vibrate as they move. In non-polarized light, that vibration happens in many different directions. A flat, nonmetallic surface, such as water (or asphalt or snowfields), reflects some of the light waves in a uniform direction parallel to the horizontal surface. This uniform vibration is called polarization. This is the glare we perceive.

Polarized sunglasses provide a vertical filter, which blocks the horizontal vibration of the waves. Imagine holding a jump rope (the wave) between two people. You move your end of it side to side - that motion is the “vibration” along the rope. Now picture a set of blinds across the middle of the rope. If the blinds are horizontal, the back-and-forth movement of the wave will continue to move along the rope. If you switched to vertical blinds, however, they block the rope from moving.

As we bobbed around in the skiff that afternoon, we spied a translucent white alien creature. It was a few inches long and undulated in the water. The top was bulbous like a jellyfish, and below were four or five sets of lobes radiating from a single stem. Capturing the suspicious creature in a bucket for closer examination, we could see a fine, rust-colored tracery of branching lines within.

I tried taking a few mug shots with my camera after we released it, but the surface of the water created a faint white glare that obscured details. (Looking at a lineup in a field guide later, we discovered it was a lion nudibranch, a type of sea slug.) My spying partner had a polarizing filter on his camera lens. One way, the filter didn't block the polarized light, so the glare on the water showed. Rotated 90 degrees, the glare disappeared. With the glare gone, the blues in the photos seem richer and deeper.

I did find it difficult, at times, to look through binoculars while wearing polarized sunglasses. Sometimes objects appeared crisp, other times they grew blurry with odd colors around the edges; it was like trying to watch a 3D movie without the special glasses. I suspect this was a result of an interaction between the prisms in the binoculars, the sunglasses and possibly the angle of the sun. As a solution, scan for hints of wildlife without binoculars first, then remove your sunglasses to look through the binoculars.

Anglers have long realized polarized sunglasses allow them to see fish clearly under the water's surface. Turning the tables, researchers have found that some fish and invertebrates can perceive polarized light in the water. Water only reflects some sunlight; some light also enters the water. Moving from air to the denser water causes at least partial polarization.

There are various theories about how fish use their ability to detect polarized light, and research is ongoing. It may help some fish, such as young rainbow trout, spot potential prey. Some zooplankton possess an outer covering that bends light waves, much like the surface of the water. Part of the light becomes polarized, so the plankton stand out against the background to fish that can see polarized light.

Fish that migrate, such as salmon, along with some insects - such as bees - and birds may use polarized light as one of several methods of navigation. Some sunlight in the sky is always polarized as it is scattered in the atmosphere. The polarized light appears in an arc at a 90-degree angle to the sun. When the sun is close to the horizon the polarization forms a vertical arc oriented north to south. In some experiments, researchers trained fish to orient themselves according to the direction of polarized light, showing they can use it to navigate. In birds, research suggests they use the polarized light in the sky to calibrate some navigation systems, such as sensing the earth's magnetic field.

That afternoon on our research mission, a female sea lion popped her head above water. She decided to put on a show, ducking under the water and darting underneath the skiff. Perhaps we were under surveillance, or merited a sea lion security escort. She continued for several minutes, rocketing in figure-eights around and below us, barely breaking the surface to breathe. With my polarized sunglasses on, I could see her sleek brown form twisting and turning, passing barely a foot below our hull, with a speed and grace that was breathtaking.

• Beth Peluso is a freelance writer, illustrator, and avid birder.



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