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Now you see them, now you don’t!

Some marine animals are nearly invisible to the human eye, but become 

much easier to see if viewed through a polarizing fi lter. If you have ever 

put on a pair of polarized sunglasses to reduce glare, you have fi rst-hand 

knowledge about how a polarized fi lter can make things easier to see. 

Many animals, including fi sh, insects, birds, crabs, and shrimp have 

built-in polarized vision that helps them fi nd food, avoid predators, navigate, and communicate with mates. Scientists know very little about how 

animals see in the deep ocean, and it’s quite possible that many animals 

have been able to avoid being seen by deep ocean explorers. Maybe some 

animals can’t be seen at all with ordinary human vision, even if they are 

right in front of us! 

The Ocean Explorer Deep Scope Expeditions looked into the deep ocean 

with new eyes, including high-tech cameras that can see animals under 

extremely dim light, as well as instruments to observe animals that make 

their own light (bioluminescence) or use various types of polarization 

vision. Here are some simple experiments you can do to start exploring 

with polarized light.

What You Will Do

Experiment with polarizing

 filters, and find out whether you 

have polarization vision

A gallery of images of plankton viewed between two polarizers. In this particular setup, a normal transparent object would still be invisible. However, if the object, or animal, contains any 

birefringent (doubly refractive) substance, it will appear quite bright. Tissues such as muscle 

and tendon can be highly birefringent and are responsible for almost all the bright areas on 

these images. Photo composite: S Johnsen using images from E. Widder.

Do You Have Twisted Vision? Do You Have Twisted Vision?
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What You Will Need

❒ Two pieces of polarizing fi lter material from 

an old pair of polarizing sunglasses, or you 

can buy polarizing fi lters from educational 

supply companies (https://www.teachersource.com; about $65 for a 15 inch x 17 inch 

sheet). The material will be marked with an 

arrow that shows the “polarization axis.” If 

you cut the fi lter into pieces, use a felt tip 

marker to draw an arrow pointing the same 

way on each of the pieces.

❒ Plastic protractor, drafting triangle, and/or 

clear plastic fork

Warning

NEVER look directly at the sun, 

even through polarized fi lters!

How to Do It

1. Shine a light through a single piece of 

polarizing fi lter material. Some of the light 

will be absorbed by the fi lter, but it probably 

won’ t matter very much how the fi lter is 

rotated. Take the fi lter outside, and look 

at blue sky, away from the sun. Now, if you 

rotate the fi lter the sky will appear darker 

and then lighter. Most light passes through 

when the fi lter is lined up with the direction 

of the polarized light.

2. Put a second piece of polarizing fi lter material 

on top of the fi rst fi lter and shine a light 

through the combined fi lter. When one of the 

fi lters is rotated, you will see that most light 

comes through when the polarization axes of 

the fi lters are pointing in the same direction 

(parallel). Less and less light passes through 

as the angle between the polarization axes 

increases, to a point at which almost no light 

is transmitted when the polarization axes are 

perpendicular to each other. 

3. Place a plastic protractor, drafting triangle, 

or clear plastic fork between two sheets of 

polarizing fi lter material and shine a light 

through the stack. When one fi lter is rotated, 

bands of color will appear and move over 

the surface of the plastic object. Flexing the 

object may reveal stress lines in the material. 

This is an example of birefringence, which is 

what happens when materials bend polarized 

light in diff erent directions. Birefringence 

occurs in many living tissues, and can make 

food organisms much more visible to marine 

animals with polarized vision.

 Note: Not all plastic objects show 

birefringence. If some of your plastic objects 

don’t work, try some others!

4. If human eyes could detect polarized light, 

you wouldn’ t need the second fi lter in these 

demonstrations. But some humans do seem 

to have very limited polarization vision. To 

fi nd out if you are one of them, try this: Face 

blue sky, away from the sun, and concentrate 

on the center of your visual fi eld (the area 

light source

polarizing fi lters parallel 

to each other

polarizing fi lters perpendicular 

to each other

The Effect of Polarizing Filters
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energy particles are all vibrating in the same 

direction, the light is said to be polarized. Most 

light, including light from the sun, from ordinary 

light bulbs, and from candles is not polarized. 

Unpolarized light can be transformed into 

polarized light in several ways, including passing 

light through a filter that only transmits light 

waves that are vibrating in a single direction. 

Light waves vibrating in other directions are 

blocked. 

It may be easier to understand this kind of 

polarization by imagining a picket fence with a 

rope passing between the pickets. If we raise and 

lower one end of the rope to make a wave, it’ s 

easy to understand that the wave can only pass 

through the fence if the wave is parallel to the 

pickets, like this: 

A wave in any other direction would run into the 

pickets and be stopped. The same thing happens 

in a polarizing filter, except the molecules of the 

filter material are lined up instead of the pickets 

in the fence.

The picket fence also helps understand what happens when light is passed through two polarizing 

filters. If the rope passes through two picket fences, 

a wave in the rope will only be transmitted if the 

pickets in both fences are parallel. If one of the 

fences is rotated, the wave will be blocked, like this:

So, if we hold two polarizing filters in front of a 

light source, and then rotate one of the filters, we 

will see the light grow brighter as the molecular 

“pickets” in the two filters become parallel. Light 

can also be polarized by reflection from nonmetallic surfaces such as roads, snow, and water. 

The amount of polarization depends upon the 

type of surface and the angle at which the light 

approaches the surface. Glare from these surface 

can be reduced or eliminated by polarizing filters 

(such as sunglasses) whose molecular “pickets” 

are not parallel to the vibration direction of the 

reflected light waves. Light can also be partially 

polarized when it is scattered off of particles in 

the atmosphere.

of sky that you can see). If you see a fuzzy 

yellowish horizontal bar or bow-tie shape, 

you are seeing “Haidinger’s brush,” and 

you have a limited form of polarized vision! 

You can also try to see Haidinger’s brush 

by looking at a bright background (such 

as a well-lit white wall) through polarized 

sunglasses. 

5. Why do you suppose so many invertebrates 

have polarized vision while humans do not? 

Part of the answer is that the human eye 

consists of a single lens that focuses an 

image onto light-sensitive cells in the retina 

that transmit nerve signal to the brain. Many 

invertebrates have compound eyes made of 

hundreds (or thousands) of “simpler” eyes 

called ommatidia. Each ommatidium has a 

lens, crystalline cone, and visual cells containing rhodopsin. In vertebrates, the visual 

pigment molecules are randomly oriented, 

but in many invertebrates these molecules 

are lined up in the same direction. This makes 

it possible for these animals to detect polarized light.

What’s Happening

We can imagine light as vibrating particles of 

energy moving in a series of waves, sort of like 

the particles of water that make ocean waves. 

In some light waves, the energy particles are 

vibrating in many different directions. These 

light waves produce unpolarized light. If the 

“rope” wave passing through parallel picket fences

“rope” wave blocked when one fence is rotated
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Want to Do More?

One of the most famous examples of polarization 

vision is the discovery by Karl von Frisch that 

bees detect patterns of polarized light in the sky, 

and communicate directions to each other by 

dancing in a specific pattern. 

• For more information about Karl von Frisch 

and his bees, visit polarization.com/

bees/bees.html.

• For more information about Haidinger’s 

brush and polarization vision in humans, visit polarization.com/haidinger/

haidinger.html and theconversation.com/

polarised-light-and-the-super-sense-youdidnt-know-you-had-44032

• For more information about the Ocean Explorer Deep Scope Expeditions, visit:

– oceanexplorer.noaa.gov/explorations/05deepscope/welcome.html

–“Secret Communication Channels in 

the Ocean: Polarization Vision” 

oceanexplorer.noaa.gov/explorations/04deepscope/background/

polarization/polarization.html

–“Hiding in Plain Sight: Birefringence” 

oceanexplorer.noaa.gov/explorations/04deepscope/logs/aug15/

aug15.html

–“Measuring Vision in Crustaceans” 

oceanexplorer.noaa.gov/explorations/04deepscope/background/

vision/vision.html

Two images of the same leptocephalus eel larva. The top image 

is viewed under unpolarized, transmitted light. The bottom image is 

viewed under polarized, transmitted light by a camera with a polarizing filter. The increased visibility 

of the bottom image is due to the 

presence of birefringent muscle 

and connective tissue fibers. Photo 

composite: S Johnsen using images 

from E. Widder.

This Caranchid squid, about four-inches across, uses transparency 

to hide from potential predators. Open-water divers can more 

easily observe these creatures with polarizing filters. Compare the 

polarized and unpolarized images to one another. Images courtesy 

Edie Widder.

This activity is adapted from “Twisted Vision,” a lesson from the Ocean Explorer Operation Deep Scope 2005 Expedition (oceanexplorer.noaa.gov/explorations/05deepscope/background/edu/media/twisted.pdf); by Mel Goodwin, PhD, Marine Biologist and 

Science Writer, Mt. Pleasant, SC.