3.1

Polarization and liquid crystal

First of all, the polarization of light is explained using a few slides. Then, we also show a plastic polarizer to students and explain why the polarizer looks dark. In general, light from the sun or light bulb is not polarized and has a variety of polarizations. The plastic polarizer lets one polarization of light pass, and reflects light with other polarizations. With two polarizer perpendicular to each other, we demonstrate that light is completely reflected, and therefore it looks dark. We pass the polarizers around so that students can play with them as shown in Figure 3.

Figure 3:

(a) Two plastic polarizers. (b) Structure of LC cell (above) and a box that emits polarized white light (below). (c) Demonstration of liquid crystal cell with a polarizer on top. (d) Students make their own liquid crystal cell.

Before discussing liquid crystal (LC) displays, we emphasize that liquid crystals are everywhere and that in fact we use them in the display of mobiles, laptops, calculators, watches and so on. With a drawing in a slide, we show that liquid crystals consist of long molecules in liquid, and with application of voltage or heat one can align them in such a way that they transmit or reflect light with certain polarization and wavelength of light.

While showing a slide that presents the structure of a LC, we show a LC cell. As shown in Figure 3 (b), this LC cell has E7LC [4] sandwiched by 2×2 cm polyethylene terephthalate (PET) plates, and is glued to keep the LC between the plastic plates. When the cell is placed on the light source with one polarization, still the cell looks transparent. However, upon placing a polarizer on the other side of the LC cell, it shows different colors depending on the angle of the polarizer in the plane parallel to that of LC cell as shown in Figure 3 (c). Students learn that due to the orientation of LC molecules the polarization is rotated differently for different colors or wavelength. Thus one can change colors by changing the angle of the polarizer. Moreover, we explain that in LC displays, each pixel, which corresponds to a LC cell, changes its color by voltage instead of rotating the polarizer. This is because voltage changes the orientation of the LC molecules, which rotates the polarization of light with specific color, and enables us to extract specific colors by tuning voltage.

After a talk, we give students LC kits provided by the OSA Rochester section [5]. Students make their own LC cells that change their color depending on the temperature to understand that a certain temperature can orient the LC to reflect certain colors of light. They are amused and excited by learning what a LC is and how it works. They put the cell on their hands and compare the difference of temperature on their body with that of other students. The tools used for this demonstration are listed in Table 1.

Table 1:

Materials for the experiment of polarization and liquid crystal.

3.2

Reflection and refraction

With a triangle prism, we demonstrate refraction and reflection of light (Figure 4 (a)). When red light from a laser pointer is directed to the prism, students witness the light is bent the interface of air and prism, and thus they learn about the concept of refraction and the refractive index of materials. Moreover, depending on the angle of incidence, light that travels in the prism passes through the other side of the prism and goes out, or the light is reflected at the other interface of the prism. In this manner, the total-internal reflection of light is demonstrated in a simple experiment.

Figure 4:

(Color images) Reflection and refraction of light by (a) a laser pointer and prisms, (b) optical fibers, (c) a box with mirrors, and (d) a cup with water and the inset is the fountain of Montjuic.

After understanding reflection and refraction, students are exposed to various applications of the phenomena. As in Figure 4 (b), we present optical fibers that connect long-haul communication system in which the data that encodes their voice may be carried as a form of light signal when they call someone in the North America from Europe. We stress that information in internet is carried in the same way as well. In order to understand the guiding of light by total-internal reflection, we let student play with a laser pointer and a foam box that has a hollow space with mirrors on both sides of the hollow space as shown in Figure 4 (c). With this simple gadget, the guiding of light by a series of reflection is visualized for students, and they learn how light from one end of the fibers propagates and comes out from the other end of the fibers.

We also perform an experiment on the guiding of light in flowing water. A plastic cup with a small hole in which water comes out is used. One of our volunteers shines a laser pointer toward the hole and demonstrates the red light of the pointer follows the path of flowing water. They are also explained that the fountain of Montjuic (See the inset in Figure 4 (d)), a famous touristic spot in Barcelona where the water of the fountain changes its color with music, works in the same principle as fiber optic cables.

As another everyday application of reflection, a volunteer explains optical data storage, such as CDs and DVDs that utilize reflection of laser to read information from the disc. Students learn that there are pits on disc surface which reflect laser to the detector in a different way depending on the shape and location of pits. In this manner information from the disc is optically read in terms of the sequence of the pits. The required kit for this session is shown in Table 2.

Table 2:

Materials for the experiment of polarization and liquid crystal. Note that optical fiber cables do not necessarily have to be the ones for telecommunication. For demonstration purpose, optical fibers for decoration are sufficient.

3.3

Stereoscopic vision

We perceive 3D image as superposition of two slightly different images obtained from both eyes. With one eye it is hard to obtain information about the distance of the image, though we can see the image in 2D. To demonstrate this concept, one volunteer asks a student to close one of his or her eyes and try to touch the finger of the volunteer from the side. Most of the students find themselves unable to touch the finger of the volunteer, and realize that it is hard to perceive the distance to the finger with one eye. Moreover, a volunteer asks a student to look at two rings of metal wire with one eye as in Figure 5 (a) and to tell which ring, either small or bigger one, is closer to the student, which turns out to be quite challenging to distinguish with one eye. Thorough those experiments, students realize that it is handy to have two eyes in order to comprehend the sense of distance.

Figure 5:

(a) Two rings of metal wire. (b) Demonstration of stereoscopic vision with 3D glasses. (c) An example of 3D pictures [6].

Then, each student receives 3D images and 3D glasses. In the easiest form, these 3D pictures are essentially two blue and red pictures that are put together but slightly shifted when they are printed on a sheet of paper. Suppose the 3D glasses are such that the right eye see through a red filter and the left one a blue filter. Since these two colors are complementary, it is then possible to include the two pictures on one single 3D picture and print it on a piece of paper. The red picture will only be seen by the left eye with the blue filter. The reason for this is that the right eye with the red filter does not distinguish between red and white. Hence the eyes see two pictures with slightly different angles of the same scene, which enables our brain to give us a sense of depth and a sense of stereoscopic vision. Because of the complementarity of these colors, the same principle also applies to movies. Apart from the internet, we got the 3D pictures through a collaboration with a local photographer who specializes in 3D imaging [6]. The required kit for this session is shown in Table 3.

Table 3:

Materials for the demonstration of stereoscopic vision.

3.4

Concluding remarks

After the demonstrations, students, teachers and volunteers gather and review what the students learn, by asking the three question that we asked at the beginning. Then, we take 5 minutes to get feedback from students who write their opinion on sheets of papers so that we can improve our presentation. Some of the high school teachers who came along with students asked about the materials used in demonstration so that they could perform some of the experiments at school. We also provided them with take-home materials, such as liquid crystal cells and 3D pictures.