Fun With Light

Polarized Light

I recently purchased a sheet of polarizing filter material so I could play with polarized light and show the effects to my granddaughter, Sophie.

Light propagates through matter by causing electrons within matter to oscillate. Electrons absorb light and oscillate as a result. Oscillating electrons then emit light. Light transmission proceeds through a series of absorption and subsequent emission events by one electron after another along the path of transmission. Molecules in the polarizing filter have a structure such that electrons can oscillate in one direction, say up and down, much, much easier than they can oscillate in a perpendicular direction, say side to side. When unpolarized light, oscillating in all directions, propagates through the polarizing filter, up/down oscillations get through much, much easier than side to side oscillations. Light exiting from the polarizing filter is, therefore, oscillating almost entirely in the up/down direction, and, thus, becomes mostly linearly polarized in the up/down direction. 

I found my laptop screen to be a good source of white light. For example, a blank new file in Microsoft Notepad looks like the first image below.

I probably should have known beforehand that laptop screen light is polarized, but I discovered it with the polarizing filter. The next image shows a piece of polarizing filter held so it lets laptop light through.

The next image shows the filter rotated 90 degrees, completely blocking light from the screen. So, laptop light is linearly polarized!

Next, I tried the well-known procedure of placing various plastic objects between the polarized laptop light source and a polarizing filter. I thought Sophie would like seeing how colors appeared like magic. For example, the next image shows an ordinary empty tape dispenser placed between the computer screen and the polarizing filter.

Next, I tried the bottom of a plastic McDonald's cup.

Sophie said, "Cool!" I was thrilled that she liked it! But what causes the colors? The light source is white, and the polarization filters alone don't cause colors to appear. It must be caused by the properties of the plastic items themselves and how they interact with polarized light.

The colored plastic material is birefringent. This means the internal molecular structure creates asymmetric conditions for the propagation of polarized light. (In other words, electrons can oscillate more easily in one direction than another.) In one particular polarization direction, say the x-direction, light travels fastest through plastic. If light is polarized in the y-direction, perpendicular to the x-direction, it travels slower through plastic than light polarized in the x-direction. This is equivalent to saying the index of refraction varies with polarization direction.

When polarized white light from the laptop enters the plastic it encounters molecular asymmetries mentioned above. Some component part of the incident polarized white light encounters regions where it travels fast, and some other component part encounters regions where it travels slower. The components get out of step and finally emerge combined from the plastic with an overall different exit polarization direction than the entrance polarization direction. The change in polarization direction depends on the wavelength of the light, therefore different colors change polarization direction by different amounts. So when we view the plastic object through a polarization filter that would block polarized laptop light, we don't see blackness. Instead, we see a range of colors where different wavelengths of white light have changed their polarization direction by different amounts.

I tried several other items with mixed results. Some looked very nice, like this protractor from my old college drawing kit.

The (broken) circle template was also interesting.

This French curve drawing tool was nicely colored.

I rummaged around the house looking for other plastic candidates. An old cassette tape case produced a good result.

Would an old CD case also show colors? Yes.

How about the plastic lid to a bathroom Q-tip dispenser?

I tried an empty sliced ham container.

The last item of this type was the cylindrical plastic tube of an "energy stick".

Colorful effects can also be achieved with overlapping clear "scotch" tape segments. Sophie and I enjoyed making some of these by placing tape strips on a clear plastic sheet. Fortunately, we had the kind of tape that works. I've heard some types of tape don't work. Different tape thicknesses change polarization directions by different amounts depending on wavelength, so wonderfully colorful arrangements result.

When the polarizing filter in front of an overlapping tape arrangement is rotated a nice kaleidoscopic effect results.

All these effects are well known. There's nothing new here except the fun we had when my granddaughter saw these for the first time.