Teaching and Learning Forum 97 [ Contents ]

Polarizing effects

Geoff I Swan
Department of Applied Science
Edith Cowan University

Introduction

Students in the first year physics units SCP1112 Waves and Electricity and SCP1141 Physics for Engineering 1 at Edith Cowan University receive a lecture on the production and properties of polarized light. Almost all students have a TEE physics background and have not previously studied polarization.

The polarization lecture is the last in a series of lectures on waves. Students have already been exposed to the production and properties of waves in lectures and in the laboratory. In particular, transverse harmonic waves on a stretched string and the interference and diffraction of light have been covered quantitatively in some detail.

The introductory section of the polarization lecture seeks to develop student understanding in the following areas:

Polaroids are placed on the overhead projector to demonstrate various effects of polarization during the instruction. To understand and predict how light is transmitted through polaroids, the vector nature of the electric field has to be believed and applied by the students.

Polarization and polaroids: Pedagogy

Students are first reminded with the aid of a suitable diagram (see Figure 1) that light is an transverse electromagnetic wave. The following points are made:

Figure 1

Figure 1: (from [1]) An electromagnetic wave travelling along the x-axis consists of an oscillating electric field (E) in the vertical plane and an oscillating magnetic field (B) in the horizontal plane. This wave is said to be vertically plane polarized which is represented by the vertical double arrow on the right.

A plane of polarization can be defined for any transverse waves and it is at this point that transverse waves on ropes are introduced to reinforce the concept of polarization. The concept of a polarizing material or device is also introduced. Figure 2 shows two transverse waves on a rope; one vertically plane polarized and the other horizontally polarized. This transparency provides a more physical picture for the students to accommodate into their conception of polarization.

Figure 2

Figure 2: (from [2]) A vertical slit acts as a filter with a vertical transmission axis for the transmission of transverse waves on a rope. The vertically polarized wave (a) can pass through this filter whereas the horizontally polarized wave (b) is absorbed or reflected.

The transparency also shows the affect on wave propagation due to a vertical slit which prevents the horizontally plane polarized wave from passing through but allows the vertically plane polarized wave to be transmitted. The concept of a polarizing material or device, where transmission properties depend on the plane of polarization relative to the transmission axis of the material through which the wave is travelling, is thus introduced.

This concept can now be extended to light waves by introducing a well known material (polaroid) as a polarizer for light which essentially has the same function as a slit has for a wave on a rope. The polarizing sheet (polaroid):

Unlike waves on a string, light waves are usually unpolarized. The overhead projector provides a useful source of unpolarized light and this is easily demonstrated by placing a polaroid on the projector and rotating it through 90 degrees: the intensity of the light projected through the polaroid remains constant.

At this stage the student should have accommodated the concepts of polarization and the use of a polaroid (polarizing sheet) to produce plane polarized light parallel to the transmission axis.

Figure 3

Figure 3: (from [1]) Only vertical components of the unpolarized light are transmitted through the first polaroid (P1) producing plane polarized light parallel to the vertical transmission axis of P1. The second polaroid (P2: the analyzer) has a horizontal transmission axis and consequently absorbs the incoming vertically polarized light. No light is seen by the observer through these crossed polarizers.

The second half of this section deals with the propagation of plane polarized light through further polaroids sheets. A predict, observe and explain approach is adopted which I have found to actively engage the students' minds.

A polaroid (P1) is placed on the overhead projector. Students are then asked to predict (by invitation or a show of hands) what will happen to the intensity of the light projected through P1 onto the screen when a second polaroid (P2) is placed on top of P1, such that the:

Almost all students correctly predict the light intensity seen for each of the above situations respectively: The proportion of light which passes through the P2 depends on the angle between the plane of polarization of the incident light and the transmission axis of P2. Students need to realise that a smaller the angle between the transmission axes of the polarizing sheets P1 and P2 means more light passing through P2 as there is a larger component of the incoming plane polarized light which is parallel to the transmission axis of P2.

This is explained with a transparency which shows that the incident electric field amplitude is a vector which needs to be resolved into:

The transparency also quantifies the magnitude of the transmitted electric field and intensity:

Equations for the magnitude and intensity of the transmitted electric field

Students generally don't have much difficulty assimilating these concepts but they also choose not to use these concepts in predicting the outcome of one particular situation involving three polaroids.

Two polaroids are crossed and placed on the overhead projector. No light is able to pass through both ........ or is it? A third polaroid is produced and the students are asked to predict what happens to the light intensity when the third polaroid is placed:

Of course, nothing happens when the third polaroid is placed on top for any angle and almost all students predict this. However, when the third polaroid is placed in between the crossed polaroids, light is suddenly projected onto the screen where previously there was none (see Figure 4). This absolutely amazes most students as almost all students predict no light under any circumstances. They have a firm notion that when two polaroids are perpendicular, no light can pass through them under any circumstances. Their logic is fairly straightforward: If one polaroid transmits only vertically polarized light and another later on absorbs all incoming vertically polarized light then no light can pass through both. They have not appreciated the vector nature of the electric field in so far as the magnitude and direction can change as the light passes through each polaroid. This is explained using an appropriate diagram (see Figure 4) on a transparency.

Figure 4

Figure 4: Vertically plane polarized light produced by the first polaroid (P1) is rotated (at reduced amplitude) by 90 degrees by passing through two polaroids (P2 & P3). Each polaroid has its transmission axis rotated by 45 degrees relative to the previous polaroid.

Students have observed three polaroids in series being used produce plane polarized light and then rotate the plane of polarization into a direction where magnitude earlier on was zero! This may seem rather strange, but this is a real observation in the real world and humans don't make the rules; they only seek to know, understand and then use them to advantage (well...usually anyway!). At the end of the lecture, I've always had a few students wanting to view the polaroids again on the overhead projector - they still don't quite believe it!

Conclusions

The properties of light passing through polarizing sheets can be explained theoretically and demonstrated at the same time using an overhead projector. This generally leads to an understanding of the basic properties of polarized light with a strong connection between theory and observations.

The predict, observe and explain approach adopted gets students actively involved in a lecture environment. After the initial concept of polarization has been taught, they are asked to predict what is going to happen for various situations. For each of these stiuations, a quick demonstration is performed using the polaroids on the overhead projector which either confirms or conflicts with their existing conceptions. An explanation of each observation is given or elicited from the students. Misconceptions when they are demonstrated to be incorrect (to the student) are likely to discarded if better (and correct) conceptions are readily available.

Placing a third polaroid at 45 degrees between two crossed polaroids on the overhead projector is a powerful demonstration of the vector nature of the electric field. Most students are stunned by what they see. Many students don't fully discard their misconceptions until after this demonstration.

Acknowledgement

Thanks to previous Edith Cowan University lecturers Frank Dymond and Bob McInerney whose transparencies greatly influenced my approach to this section.

References

[1] Halliday, R., Resnick, R. & Walker, J. (1993). Fundamentals of Physics, (4th. ed). New York: John Wiley.

[2] Giancoli, D. (1995). Physics (4th. ed.). New Jersey: Prentice-Hall

Please cite as: Swan, G. I. (1997). Polarizing effects. In Pospisil, R. and Willcoxson, L. (Eds), Learning Through Teaching, p314-318. Proceedings of the 6th Annual Teaching Learning Forum, Murdoch University, February 1997. Perth: Murdoch University. http://lsn.curtin.edu.au/tlf/tlf1997/swan.html


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