Polarization

 Last week, I recorded a video about the polarization of waves, especially light waves. One of the best ways to learn about polarization is to experience it!

Get Polarized!

Pick up some supplies like a pair of polarizers or some sunglasses to try out the demos yourself. After learning about polarization demos and labs in this article and my video, you will probably want to order a class set of polarizers. 

Every student should engage with these polarizing filters in a hands-on manner.

 The behavior of polarizers is that either one will let some light through, even when they overlap, but when they are crossed, they do not let light through.

 What can we learn about light from using polarizing filters?

We can learn that light travels in transverse waves. Imagine a transverse wave like the ones on a slinky. There is up and down and left and right and all mixed up. But I can polarize the wave and allow it to travel in just one plane. No matter what crazy wave I try to send, only the horizontal components will get through. This is what is happening to the light when we use a polarizer and it proves that light is a transverse wave.

Regular light, from the sun or a flashlight, is randomly unpolarized, but sending it through a polarizer polarizes it. In some cases, we have vertically polarized light. Sending this through a second polarizer, often called an analyzer, might not permit any light to pass, especially if the polarizers are at right angles. Cancelling light by crossing polarizers is called extinction.

My video will show a very cool demo using three polarizers. If we have two polarizers crossed to extinction, and we add a third polarizer in between, we still get light to pass through them.

Why Should You Do This Demo?

What does it show? This experiment helps us understand what it means for something to be polarized. It means that it is oscillating in a specific orientation, as a vector.

A vector can be broken down into its components. It is these components of the light that oscillates diagonally that get through the triple polarizer. Therefore, the diagonal polarizer allowed these components to pass through, and the third polarizer let the components of those components to pass through.

See the World Differently…through Polarizers

Did you know that Liquid Crystal Displays or LCDs are polarized?  You can have a lot of fun by putting a polarizer in front of your computer screen which is probably an LCD screen. These screens are linearly polarized usually diagonally. Now try it out on your watch or your calculator!

Reveal the “Stress” these Everyday Items are Under

Crossed polarizers can reveal the amount of stress in a transparent object. For example, the photo of a  polarized protractor, above, demonstrates that in some places there is more stress than others.

The revelation is made by the medium repolarizing the light in between two polarizers that have already been crossed to extinction. You can do this by stretching transparent tape, too. Next take a look at rulers or plastic silverware or the safety goggles in your lab. 

In my video, I will even show a shattering “Prince Rupert’s Drop.” 

Nice Shades: Reducing Glare

One of the main purposes of polarizers is to eliminate glare. How does this work?  Well, glare is either partially or completely polarized, whether it is glare on the surface of your black lab tables or glare on water puddles.

Glare tends to be made of polarized light that is oscillating in the same plane as the surface that is reflecting it. For example, the surface of the black lab tables have glare that is polarized left and right.

The Sky

The blue sky is a glare from the sun scattered by air molecules and it is partially polarized. The polarization is at a maximum at positions that are at 90 degrees from the sun arms. 

Go outside and rotate the polarizer; you should notice that the sky gets darker as you rotate it. Photographers sometimes use this effect to make the clouds look more dramatic. You can try using it for fun and to better see the moon during the day.

Iceland Spar

The crystal above is called Iceland spar. It is a type of calcite, a metamorphic rock from Iceland that possesses the unique property of double refraction. 

There are two different refractive indexes and each of the images has a different axis of polarization.

Before artificial polarizers were created, Iceland spar was the original polarizer. It was Iceland spar that convinced Augustin Fresnel that light was not just a wave, but a transverse wave. When a crystal like this one is cut and made to act as a polarizer it is sometimes called a Nicol Prism.

If you go to a 3-D movie, save the glasses!

If you want to know more about what 3D glasses and polarizers have in common, you will just have to wait (in total suspense) for my video. In the meantime, start collecting 3D glasses for your classroom. 

The plastic film can be removed from the surface of the glasses, and what remains is just a linear polarizer. If you do not yet have a class set of little polarizers, you should definitely start collecting 3D glasses. Maybe even offer some extra credit to your students.

**In my next post, I will discuss how you can use Polarizers to analyze the Faraday Effect and the Brewster Angle.