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In this activity, we are going to take advantage of some cool things about light and how it interacts with different materials to make art. To understand how this activity works, we need to learn a little about light and the materials that will affect how you see it – polarizing filters and cellophane.
Let’s learn about light
Light is a wave. More specifically, it is an electromagnetic wave, consisting of electric and magnetic fields, denoted as E and B, respectively, in the figure below. Since light is a wave, these electric and magnetic fields are moving back and forth over and over again, also called oscillating. One amazing aspect of light is that these electric and magnetic fields are always oscillating perpendicular to each other and propagating themselves forward, or in the z direction below, at, well, the speed of light!
In order to fully understand the science behind this activity, we need to note two characteristics of light.
- The first is the wavelength, denoted as λ (lambda) above, and refers to the distance between the two closest identical features in a wave. For light, the wavelength determines its color, with 700 nanometer (nm) light looking red and 400 nm light looking violet.
- The second is the direction that the electric field is oscillating, known as the polarization. In the figure above, the electric field is oscillating up and down, so we would say it is polarized up and down.
Let’s learn about polarizers
Light, like from a flashlight, consists of many electromagnetic waves with different polarizations. In the figure below, the arrows represent the direction of polarization. A polarizer (like you might find in expensive sunglasses), blocks out light that is polarized in all directions except those that match the polarizer, polarizing the light.
Figure Credit: Matthew D. Stilwell
Most commercial polarizers are made using polyvinyl alcohol. Polyvinyl alcohol is a polymer, meaning that its molecules are long chains of repeating units. The polyvinyl alcohol polymer is stretched during manufacturing, causing the long, molecular chains to align in the direction they were stretched. Iodine is then added to the polymer, causing the molecular chains to conduct electricity along their length. When light passes through the polarizer, only the light waves with electric fields oscillating in the same direction as the conductive polymer chains make it through, with the rest being absorbed.
Let’s learn about cellophane
To understand how the light interacts with the cellophane, we need to understand a characteristic of materials. This characteristic is called the refractive index, which describes the speed at which light travels through it, which in most cases is more slowly than in air. The refractive index varies based on the wavelength of light. One place you may have seen the refractive index varying by wavelength is in a rainbow or in a prism, like the one below.
Some materials, like the cellophane, are birefringent, meaning that its refractive index depends not only on the wavelength of the light, but also on its polarization. As light enters a birefringent material, it resolves into two components. In the image below, you can see blue light passing from the left to right through a birefringent calcite crystal and being split into two beams. The refractive indices that these two components experience differ, so the light becomes “phase shifted.” This phase shift causes the light leaving the material to have a different polarization than when it entered!
Putting it all together
In this activity, we start with white light, like from a flashlight, which contains all the colors of light. This white light becomes polarized as it passes through the first polarizer. Next, the polarized light passes through the birefringent cellophane, becoming phase shifted and leaving with a different polarization. Since the refractive index of the cellophane depends on both the polarization and the wavelength of light, different colors phase shift a little differently, causing their polarization to change differently, too.
Figure Credit: Matthew D. Stilwell
Depending on the thickness of the material and the wavelength of light, this “phase shift” can also cause the light to interfere with itself such that the light is extinguished. When we view the light passing through the second polarizer, at certain angles we see individual colors. We see the individual color, for example yellow, because the complementary colors, in this case purple colors, were extinguished due to the phase shift, leaving us to enjoy the beautiful yellow light.
- Two Polarizing Filters, approximately 75 mm x 75 mm
- Alternatively, you can use two pairs of polarized sunglasses
- Or, one pair of polarized sunglasses and a polarized light source like a phone screen
- Cellophane, approximately 150 mm x 150 mm is needed, but depends on how large you want your art to be!
- Clear shipping/packaging tape can also be used
- Clear glue (e.g. Elmer’s school glue) (optional)
Safety & Procedure
Do NOT look directly at the sun or at laser light sources, even through polarizers.
- Stack the polarizers and hold them out in front of you. Look through them.
- What do you see?
- What happens when you rotate one of the polarizers?
- What happens when you look through one polarizer at a phone, computer, or TV screen?
- Next, we’ll explore what happens when we put clear materials between the polarizers.
- Place a corner of the cellophane between the two polarizers. What do you see? What happens when you rotate a polarizer?
- Look for other types of thin clear plastic around your house (plastic bags, cling wrap, the clear window in an envelope, etc.) and look at them between the polarizers. Record your observations.
- Fold some cellophane on top of itself to create an area that is one layer thick and another area that is two layers thick. Place it between the polarizers so that you can see both areas. What do you see? What happens when you rotate one of the polarizers? What happens with three or four layers? Record your observations.
- Now, you can make art using the colors you saw for different layers of cellophane! You can fold the cellophane or cut it and lay pieces on top of each other to create a picture. Glue or a small drop of water can hold the layers together. Put your art between the two polarizers and try rotating one polarizer. What do you see?
Below are a few examples of art created using this method to give you some inspiration!
Photo Credit: Aedan Gardill
Dally, J. W., & Riley, W. F. (1965). Experimental stress analysis. New York: McGraw-Hill.