What is the meaning of Polarization?

Polarization is a quality that is only exhibited by transverse waves. For light waves, the polarization identifies the direction of vibration of the electric field of the light. Any beam of light can be resolved into pieces that have polarizations at right angles to each other, as, for example, in vertically and horizontally polarized light. A polarizing filter will block light that is polarized at 90o from its polarization axis. If you have randomly mixed light, a polarizing filter will block half the original intensity of light. If you pass light through two filters with their axes at 90o from each other, no light will be able to pass through.

A way to think of these polarizing filters is that they are like vertical or horizontal slots that may or may not allow vibrations of a rope to pass through them, depending on the direction of the filter compared to the direction of the vibrations. If we start with a filter with vertical slots (vertical polarization axis), then any horizontal vibrations will not make it through. If you then add another filter with horizontal slots, the vertical vibrations that made it through the first filter will now be blocked by the second filter.

This gives you a method for testing whether the sunglasses at your local drug store are polarized or not. If two pair of sunglasses are polarized and you hold one pair at 90o from the second pair, then no light should be seen through the overlapping lenses. Polarized sunglasses are good at eliminating glare because they will not transmit horizontally polarized light. Most light that is reflected from puddles in the street or large bodies of water is horizontally polarized.

If you have an old pair of polarized sunglasses that you no longer can wear, there are a couple of fun experiments that you can do with polarization of light. First, find a clear plastic fork or something else made of clear plastic like a tape dispenser. Put this object between two polarizing filters at 90o to each other. Just the polaroids alone will let no light through, but once a transparent plastic object is placed between them, you will see ribbons of color that indicate variations in the thickness of the plastic which twist the polarization so that light can get through. These bands will change if you squeeze the tines of the fork. This is a variation of Newton’s rings and is used in the optical industry to measure the flatness of a surface to high precision. This can also be used to analyze the placement of stresses in structures. A similar effect can be seen with very cheap clear tape (should not look frosted) that is not Scotch™ brand. The cheaper the better - I have found that Toys R Us™ tape works well. The reason it needs to be cheap and usually not as good as Scotch tape is so that it is very thin plastic. If you take a piece of clear plastic like a transparency and put a mosaic of cheap clear tape on it, then place this between your crossed polarizing filters, you can see a pretty color mosaic if you hold this up to the light. These are like the colors seen in soap films.

If you have a digital watch you have a polarizing filter on your wrist. The way that liquid crystal displays actually display numbers is that the orientation of the long crystals in the liquid changes when a voltage is applied. So that if a segment has a voltage applied, then it becomes a polarizing filter at 90o and the backlight of your watch does not pass through. How can you test this if you have a pair of polarizing sunglasses? Try it first then check your answer.

If the polarizing filter is oriented in the same direction that the LCD becomes polarized, no light will be able to penetrate and the entire display will look black. This gives you another way to check the sunglasses at the drug store!