Physics 2000 Science Trek Polarization


I'll answer that, Kyla, but first we're going to have to talk about polarized light. Think back to when we talked about electromagnetic waves. We learned that the electric force field moves up and down as the electromagnetic wave moves forward. The light is brightest when the blue electric force arrow is biggest, and it is dark where it is zero.

It still seems strange to think about a force field that moves in a different direction than the light, but I guess light is just one example of the electromagnetic waves we talked about earlier.

You're exactly right, Kyla. In general, the direction the wave moves is called the "ray" direction. This "ray" direction points along the path of light "rays".

You mean like the light rays coming through a window into a dusty room?

Yes. That and most light is NOT polarized, although the electric forces still move up and down perpendicular to the ray direction. In the picture above, the light is polarized in the plane of the yellow arrow. Unpolarized light looks more like the movie below.

It looks like the yellow arrow plane keeps jumping to a different angle, even though the ray direction remains the same.

That's right, but notice that no matter how the yellow arrow twists, the electric forces are always perpendicular to the ray direction. In unpolarized light the twist of the yellow arrow plane keeps changing randomly. We will use the picture below on the left as a symbol for polarized light, and the picture on the right as a symbol for unpolarized light.

Polarized Light Unpolarized Light

If the yellow plane of polarization is always twisting in unpolarized light, how is it possible to transform unpolarized light into polarized light where the plane doesn't change?

That's a good question, Kyla. The electric force field in any plane of light can be separated into a vertical and horizontal component, so you can think of a diagonal plane of light as being made up of some vertical polarized light and some horizontal polarized light. A good way to picture this is to imagine pushing a heavy box.

You can push the box along the diagonal by yourself, but you will have to push pretty hard to move it. On the other hand, you can get a friend to push it to the right while you push it forward, and the box will end up in the same place. Because the two of you are pushing together, neither one of you will have to push as hard as if you were pushing alone.

So you are saying that we can think of the electric forces in a light wave in the same way?

Yes, the electric forces in one yellow plane of polarization are completely equivalent to the electric forces in a vertical yellow plane PLUS the forces in a horizontal yellow plane, just as below. This is called "breaking the light up into horizontal and vertical components".

You can always imagine breaking light into horizontal and vertical (polarization) components. This is true whether the light on the left is polarized or unpolarized, but temporarily polarized in the plane shown on the left.

Now I understand what polarized light is, but I guess I still don't understand how it gets polarized in the first place. Why were we able to polarize light with the sunglasses?

In order to polarize light, you need to pass it through some kind of filter. A good example of this is a Polaroid filter. This kind of filter is made up of parallel strands of long molecules. Let's think of a lens where these strands are horizontal. The energy of the horizontal components of light is absorbed by the strands, so this part cannot pass through. The vertical components can pass through, however, because the horizontal strands cannot absorb their energy.

So the filter selects one component from all of the different planes of light and lets that one component get through! That's why light that's polarized in the horizontal plane can't get through a filter that's absorbing the horizontal components of the light.

You've got it, Kyla. We've seen what happens to light when it passes through one lens, but more exciting things start to happen when it passes through a few lenses...

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