Light travels in waves, but unlike waves on the ocean, they are much too small for us to see with the naked eye. A polarizer is a light filter that only allows light waves that are moving in one direction to pass through, letting us indirectly observe some of the wave properties of light. This aligned light that passed through a polarizer is called polarized light.
Sunlight is Unpolarized
Direct sunlight is unpolarized, meaning that the waves travel in many random directions. When sunlight passes through a polarizer, only the waves that are aligned in the direction of the polarizer filter are able to pass through. Hold up your polarizer filter to the sky. You’ll notice that a polarizer filter looks dark, and that is because it is only letting through the aligned light waves.
Reflected Light is Partially Polarized
Now rotate your polarizer and watch some objects like cars, windows, or other shiny objects. You should notice that some of the reflections disappear. While sunlight is randomly polarized, once it reflects off a surface, it tends to become polarized because of how the light waves interact with the surface. This will depend on the type of surface and angle of reflection, and can be very complicated. However, we can still see the effects of these nearly invisible waves through the polarizer.
Sunlight and the Earth’s Atmosphere
When you look up at the sky with a polarizer, there is something even more subtle happening. Observe how deep the color of the sky is while you rotate the polarizer. You will likely notice a change in brightness and color of the sky, and you may notice that clouds are more visible at times. This is because the sunlight passes through our atmosphere and reflects off air molecules, and also becomes slightly polarized. This will vary quite a bit by time of day, mostly due to the angle of the sun. Photographers have been using polarizers like this for decades to create vivid outdoor pictures.
If you have a cell phone camera, you can place the filter in front of the camera and try this yourself. Take pictures of the same scene while rotating the filter and see if you notice a color and contrast change. Important hint: there is an angle where light is strongly polarized, called the Brewster's angle, and if you are at this angle the effect will be very easily noticeable as in the 2 photos below. This angle depends on the surface among other things, but about 53 degrees will be close.
Are Lasers Polarized?
Some are polarized, but not all. It depends on the light source and how the optics are designed. With your polarizer filter, you can test this. If you have a laser pointer, shine the light through the polarizer and rotate it. Observe the laser spot on a piece of white paper (in a darkened room will work best). Does the brightness of the spot change when you rotate the polarizer? If it does, your laser is partially polarized. If the laser spot almost disappears, it is mostly polarized. We can say 'partially', 'almost', and 'mostly' because perfectly polarized light is difficult to create - it requires a very good light source, very good optics, and a vacuum because even air molecules can scatter light and create polarization.
What is Extinction Ratio?
Extinction ratio probably reminds you of Jurassic Park, however, it is also an optical term and it means almost the same thing. Extinction of light measures how much light is eliminated. For example, atmospheric extinction which means the amount of light blocked by the atmosphere. In our case we are causing extinction by filtering.
Take two polarizers and place one on top of the other, and then rotate one of them. You’ll notice they go from light to almost completely dark. What is happening? When the two polarizers are aligned, they let light through, but when they are misaligned by 90 degrees, they will block almost all light. The first polarizer is only allowing waves in one direction to pass, and they are then blocked by the second polarizer which only let waves pass aligned to the opposite direction.
If you align the polarizers so the maximum light goes through, then slowly rotate one by 90 degrees and it will go from maximum to minimum. The ratio of the maximum to minimum is called the extinction ratio.
Let’s Take Some Measurements
With your PocketLab or with a light meter, you can use sunlight or a bright lamp as a light source. Both of these sources are unpolarized light. You will measure the light from the lamp directly with no filter, with one filter, with both filters, and again with both filters but rotated 90 degrees. Record these four measurements. If you're using your PocketLab, the reading will be in lux, which is the SI unit of illuminance, equal to one lumen per square meter. The graph below was generated by a PocketLab Voyager, and you can clearly see four different light levels, and the recorded values are in the table below:
|2 filters (both oriented in same direction)||3,300|
|2 filters (crossed 90 degrees)||20|
First let's calculate the extinction ratio. For the data collected above it would be 17,300/20 or 865:1. We can also calculate the % transmission of just a single filter, which would be 4,000/17,300 23%. Here is a table with these two calculated values:
|Value||How is it calculated||Result|
|Extinction ratio||Maximum lux divided by minimum lux with crossed filters, expressed as a ratio||865:1|
|Single filter transmission %||Single filter lux divided by maximum lux, expressed as a percentage||23%|
Specification sheets for polarizer filters from an optics catalog (for example Edmund's Optics) show a range of transmission from 20% to 42%, and an extinction ratio from 19:1 to 10,000:1. Our data is within that range.
Some interesting questions to explore further are:
1. What makes filters with higher extinction ratios? Is a perfect filter possible, for example infinity:1 extinction ratio?
2. Most of the filters in the optics catalog are around 20%-40% transmission, and in general the more expensive filters have lower transmission. Is there a model you can draw of what may be happening to the light waves?
Match Graph Challenge
When you rotated the two polarizers, did you notice that they went from light to dark continuously? We should then be able to create any lux value between our readings in the first table; 3,300 lux with two aligned filters down to 20 lux with two crossed filters.
In the graph below, notice the approximate sine wave - can you recreate this?
Summary & Conclusions
- Light is made of waves that are too small for us to see with the naked eye, but we can indirectly observe the wave properties of light by experimenting with polarizers. Polarizers let light waves through that are aligned in one direction only, and block waves that are not aligned.
- Sunlight and light from a lamp is unpolarized, but it can become polarized when it reflects off a surface.
- There is an important angle of reflection where sunlight becomes strongly polarized, and it is called the Brewster's angle. The angle varies slightly depending on the surface.
- Laser light is an example of polarized light, but not all lasers are actually polarized - it depends on the design of their optics.