Polarized Mirages

Polarized Mirages

Polarized light from shiny surfaces looks like water to insects.


Man-made mirages. I'm Bob Hirshon and this is Science Update.

Glass, steel, and asphalt may look just like inviting pools of water to some animals. And that's messing up their lives. Michigan State University ecologist Bruce Robertson explains that certain creatures, especially insects, are attracted to a polarized light: in other words, light waves that vibrate in one particular direction. Water polarizes light, but so do smooth, dark, man-made surfaces.


And they may actually polarize light more strongly than water does.

As a result, mayflies are wasting their short lives laying eggs on buildings, while diving beetles and dragonflies are staking out breeding grounds on car hoods. If these mistakes significantly impact the bug populations, it could eventually cause a domino effect that disrupts other species. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

Today, it's easy to see that dumping chemicals in rivers, clear-cutting forests, and draining marshes can affect the wildlife that live there. But as time goes on, we're learning that even seemingly harmless aspects of our modern lifestyle have unintended consequences.

The phenomenon of “polarized light pollution” serves as just one example. First, a little more about polarization itself. You may have learned that light acts like waves in some ways, and like particles in other ways. When we talk about polarization, we're looking at light as a wave of electromagnetic energy. As that wave flows through space, the electrons that produce the wave vibrate in many different directions: up and down, side to side, diagonally, and so on. However, when the light wave strikes a polarizing surface, like water, the surface reflects back only waves that vibrate in one particular direction (parallel to the surface itself), and absorbs or scatters the other waves. That filtered, reflected light is called “polarized light.” To us, polarized light creates a shimmering effect, or glare.

As you heard, it's not just water that polarizes light. Smooth metal, glass, and asphalt also polarize light, often better than water does. This can look like water even to humans: for example, when you're driving on a long, flat road in bright sun, it sometimes looks like there are pools of water on the horizon, which disappear as you get closer.

Of course, you're probably never going to dive head-first into a parking lot because you thought it was a swimming pool. That's because humans can use many other visual cues, as well as intellectual knowledge, to tell real water from polarized light mirages. We also don't rely on polarized light to make important judgments and decisions, so our eyes and brains simply aren't strongly tuned to it. Some insects and birds, on the other hand, have an overwhelmingly powerful attraction to polarized light. These creatures need to identify bodies of water, which they may use as breeding, nesting, or hunting grounds. From an evolutionary standpoint, it's much simpler to evolve a sensitivity to polarized light than to develop brains like ours that recognize water in more complicated ways.

And for most of history, that strategy has worked. It may seem hard to believe, but it's only been within the past century, and more so in the past fifty years, that humans have had the technology and machinery to produce lots of large, smooth, shiny surfaces like skyscrapers, roads, parking lots, and cars. Robertson's work suggests that all these light-polarizing surfaces are confusing the bugs and birds, causing them to lay eggs in places where they can't grow. And while a few less bugs may not sound like such a bad thing, remember that insects make up an important part of most food webs. If the creatures that eat these insects suddenly find themselves with no food, it could start a chain reaction that severely disrupts an ecosystem.

Fortunately, Robertson says it's fairly easy and cheap to prevent polarized light pollution. For example, choosing light colors for buildings and cars, or adding white curtains to the windows of dark buildings, greatly reduces polarization. Mixing a little coarse gravel into roads can also make them polarize light less effectively, and prevent bugs from mistaking them for rivers. Robertson says it may not be necessary to do this everywhere—but it could be especially useful to think about polarized light pollution near wetlands and bodies of water.

Now try and answer these questions:

  1. What is polarized light?
  2. What kind of surfaces polarize light? Why is this a problem for some animals?
  3. Why does it matter if insects can't reproduce?
  4. Robertson says that this discovery also could be used to control insect populations, in cases where they threaten other wildlife or public health. Can you guess how?
  5. Can you think of other aspects of modern living that may affect the environment in surprising ways? 

You may want to check out the February 6, 2009, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: how polarized light fools insects, turning dolphin sounds 3-D, and tracking bird migration from space.

For Educators

The lesson plan Puzzling Polarizers, from Lawrence Berkeley National Laboratory, explores how matter polarizes light.

Blinded by the Light, from the Why Files, explores how (general) light and radio pollution affect astronomical studies, animals, and the general public.

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