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Polarizing Filters

In this exploration, students will learn how polarization works and its practical application in our everyday lives.

Light is a wave and it usually vibrates (or wiggles) in all directions. A polarizing filter is a device that allows light to pass only if it's wiggling in a certain direction. We create light that vibrates just up and down or just side to side by making it go through a polarizing filter. These filters are like bars that only let through the light that travels in the same direction as the slots.

We can use polarizing lenses to reduce glare. Light reflecting off horizontal surfaces like the road, water, or snow is horizontally polarized. A polarizing filter oriented vertically will filter out the glare, but still allow plenty of light through. A polarizing filter on your camera helps reduce shiny reflections. 

Polarizing sunglasses work the same way.

Polarizing sunglasses protect your eyes from glare off a road or water. You can tell whether sunglasses are polarizing or not. Look at a table top or water surface then turn the glasses 90 degrees. If the glare disappears and reappears when you turn the glasses, they're polarizing!

Light twists as it passes through some transparent materials: its polarization will go from vertical to horizontal or vice versa. Some colours will twist more than others. For example, blue will twist more than red.

Placing these transparent materials between two polarizing filters will result in only seeing specific colours. The first polarizing filter will only let in light that travels in a certain direction. This light gets twisted as it goes through the transparent material, changing the directions of the various light components. As this twisted light travels through the second polarizing filter, only the colour that twisted in the same orientation of the filter will make it through to your eye .

LCD (Liquid Crystal Display) screens use polarized light.

When you look at an LCD screen through a polarizing filter, you can turn the filter to make the display appear and disappear.

Objectives

  • Explain how polarizing filters work and their use in our daily lives.

  • Describe light as a wave

Materials

  • Per Group:
    a skipping rope
    2 cooling racks (with a horizontal grill, not a crisscrossed grill)
    LCD screen — optional (digital watch or monitor screen)

  • Per Student or Small Group:
    2 small pieces of polarizing filter
    *available to purchase from Arbor Scientific
    OR
    #SWatHome: Polarized sunglasses

Key Questions

  • When one filter is rotated against the other, what do you see?
  • What is happening to the light waves as the orientation of the filters changes?When a transparent object is placed between two polarizing filters, what happens?How does the light wave change as it passes through each layer?Where do the colours come from?What happens if you slightly rotate the second filter? Why?
  • Do all the transparent materials react the same way?
  • What could polarized lenses be used for? (sunglasses, photography . . . )

What To Do

Part 1: Demonstration of Light Waves

  1. Choose a volunteer and hold a skipping rope stretched out between you.
  2. Move your hand up and down to make the rope wiggle up and down, then move it side to side to make the rope wiggle in that direction, to illustrate that light is a wave and that it usually “wiggles” in all directions.

Part 2: Explaining how a polarizer works

  1. Hold up the cooling rack. This represents a magnified version of a polarizing filter.
  2. Hold the rack so that the bars are horizontal. Demonstrate the light’s path as it passes through the polarizer. Only light that is wiggling horizontally will get through.
  3. Repeat with the bars oriented vertically.
  4. Use two cooling racks to show that if the polarizers are in opposite directions, no light will get through the pair.

Part 3: Exploration 

  1. Hand out 2 pieces of polarizing filter to each student or small group.
  2. Sandwich the filters together, then hold them up and look through them.
  3. Rotate one of the filters against the other while looking through them.
  4. Place different transparent objects between the two filters and look through.

Extensions

  • Someone wants to sell you a pair of expensive polarized sunglasses for a good price. What test could you improvise to know that they truly are polarized?
  • Put your polarizing filters on either side of a clear plastic ruler or a plastic cup. Bend the ruler or the cup. The patterns you see show where there’s the most stress on the plastic.Look at a black and white liquid crystal display (LCD) such as a digital watch through a polarizing filter. Rotate the filter slowly 90 degrees.

Other Resources

Science World Bonus Video |Search for Rainbows