Polarizing effects

First year physics students at Edith Cowan University receive an introductory lecture on the nature and properties of polarized light. The properties of light passing through polarizing sheets (polaroids) are demonstrated with the aid of an overhead projector. A predict, observe and explain approach is found to be successful in encouraging students to adopt a suitable model for polarized light. Many students do not fully adopt this model until they are confronted with observations that they cannot otherwise explain.

Introduction

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:

• concept of polarization
• production of plane polarized light using a polaroid polarizing sheet
• propagation of plane polarized light through polaroids.

Polarization and polaroids: Pedagogy

• that light consists of oscillating electric and magnetic fields which are perpendicular to the direction of propagation and to each other.
• for light travelling in a particular direction, the plane in which the electric (and magnetic) field is oscillating is not fixed and indeed the electric field can oscillate in any plane which is perpendicular to the direction of propagation.
• we can define the plane of polarization as the plane in which the electric field is oscillating. It is noted that we choose the electric field rather than the magnetic field as it is the electric field which normally interacts much more strongly with matter.

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.

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):

• allows only the components of unpolarized light parallel to its transmission axis to pass through. This results in almost half the incident light being transmitted through (some losses due to absorption and reflection)
• produces plane polarized light parallel to the transmission axis (see Figure 3).

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.

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:

• transmission axis of P2 is parallel not perpendicular but at say 45 degrees to P2

• maximum light intensity (P2 parallel to P1)
• zero light intensity (P2 perpendicular to P1)
• reduced light intensity (P2 at say 45 degrees to P1)

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

• a component parallel to the transmission axis of P2 - this component is absorbed by P2
• a component perpendicular to the transmission axis of P2 - this component is transmitted through P2 without reduction in magnitude

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:

• on top of the crossed polaroids (at 45 degrees)
• in between the crossed polaroids (at 45 degrees)

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 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

References

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