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Polarization of Light & Malus's Law

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  • 0:05 What is Polarization
  • 1:32 Uses of Polarizers
  • 2:20 Malus's Law
  • 2:58 Calculation Example
  • 4:07 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain what polarization of light is, list a number of examples of how polarization of light is used, state Malus's Law, and use it to solve problems involving the intensity of light through a polarizer.

What Is Polarization?

Have you ever wondered why some lakes appear shiny and reflective, while others you can see straight down to the fish swimming below? Or how you can see through one of those shiny lakes while wearing sunglasses?

Both these things are because of something called polarization. But before we talk about polarization, we should first talk about light.

Light is a wave. Well, actually, sometimes it's a particle. And sometimes we draw it as a beam, which is like a straight line or arrow. It's kind of all of these things at once, depending on the context. Which just goes to show that the world can be a confusing place. But in any case, polarization only makes sense if we imagine light as a wave, like a line waving up and down as it moves from your lamp to your eyes.

That wave can. . . well, it can wave (or oscillate) in lots of different directions. It can wave this way, or this way, or something in between.

Most light, like light from the sun or a light bulb, is a mixed mess of light waves that oscillate in all kinds of directions. Our eyes really can't tell the difference. But this oscillation direction is called the polarization of the light.

A polarizer is a device that cleans up this mess. It takes all that mixed up light, and only lets light through that oscillates in one particular direction. You can think of it like a piece of card with slits in it. Only waves that are waving in the direction of the slits can fit through. The rest is blocked. But why would we want to do that? What's the point?

Uses of Polarizers

Polarizers are used in many ways. Polarizing sunglasses reduce the brightness of the sun by only allowing part of the light through - the light that happens to oscillate in a particular direction.

You might also notice that polarizing sunglasses remove glare from television screens and reflections from lakes. Photographers use polarizing filters on their cameras to do this on purpose. This happens because light that bounces off a lake becomes polarized after bouncing off it. The light that bounces off the fish inside the lake is polarized differently. So if you orient the slits in your sunglasses just right, you can let through the light coming from the fish and block the light coming from the surface of the lake. This can be extremely useful if you're trying to take a photo of the inside of the lake, instead of the surface.

Other uses include LCD screens, which use polarization to create their image, and radio transmissions, which are always polarized.

Malus's Law

The one downside to polarizing is that you're losing some of the light. Even if your goal is just to take a photo, the image won't be as bright, just like how sunglasses make the world dimmer. Malus's Law is an equation that tells us just how much dimmer.

It tells us that the intensity of light that passes through a polarizer, I, usually measured in watts per meter squared, is equal to the original intensity of the light, I-zero, also measured in watts per meter squared, multiplied by cosine of the angle between the light's direction of oscillation and the direction of the polarizer's slits (theta squared). That last squared is important and easy to miss.

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