We know that light is an electromagnetic wave. In its random status, the electric vector of the light waves can be in any directions perpendicular to the light propagation direction. However, after going through a “magic slit”, the electric vector of the light waves is restricted to only one direction. We call this magic slit a linear polarizer, and the light now is linearly polarized light, with its polarization direction parallel to the slit’s transmission direction. We can imagine that if the light has a polarization direction perpendicular to the slit’s transmission direction, it cannot pass through the slit.
Linear polarizer has many applications. For example, many sunglasses are made of linear polarizers. They allow only half of the fully randomized natural light pass through. Also, since a lot of glares come towards your eyes with a fixed polarization direction, sunglasses mounted at a perpendicular angle can reject largely the glares.
Your LCD screen is also highly polarized. To prove this, we can put a piece of linear polarizer in front of your screen and rotate the polarizer, you will notice the change of the brightness of your screen (Some other LCD screens use a different type of polarizer called circular polarizer, which we will cover in a separate post). In fact, polarization is used to adjust the brightness of each pixel so you can have large dynamic range of brightness.
Now think about a pair of linear polarizers. If we put them in such a way that their transmission axis are parallel to each other, light passing through the first polarizer can still pass the second one, as shown in the following picture:
However, if the two transmission axis are perpendicular to each other, light cannot pass through both of the polarizers, as shown below:
If that’s all about a pair of linear polarizers, it will be too boring. But wait a second, let’s move one step further. For example, just grab a clear plastic ruler if you happen to have one, and put it in between the pair of crossed-axis polarizers, and view your LCD or other light sources through them. Can you see this beautiful pattern?
So what is it? Why suddenly the dull polarizers becomes so colorful? Well, when linearly polarized light passes through some non-uniform media, its polarization status may change, and the change depends on many factors, such as the wavelength, the stress in the media, etc. With the change, previously rejected light can now pass through the second polarizer, because its new polarization direction is no longer perpendicular to the transmission axis of the second polarizer.
This method is widely used to check stress in plastic and other transmissive materials. For example, in the ruler we saw above, during the manufacturing process, some stress was applied and accumulated in the ruler. We cannot see the stress with just bare eyes, however, through a pair of crossed linear polarizers, the stress shows up beautifully.
Now grab some other material and put in between the polarizer pair, and find out what can make a beautiful pattern. Here we show two other examples. The first one is a zip bag between two polarizers, and the second one is a thin film plastic wrap stretched with fingers and put between two polarizers.
If you google “crossed linear polarizers stress check”, you can see a lot of beautiful images. Have fun!
A pair of linear polarizers can be purchased from eBay.
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