Episode 313: Polarisation
Lesson for 16-19
- Activity time 155 minutes
- Level Advanced
This episode requires students to develop their idea of electromagnetic radiation. Since they cannot see either the wave nature of light or the molecular structure of Polaroid, they will have to take some of this on trust. You can establish the basic ideas using analogies. The approach is non-mathematical.
- Student experiment: Using polarising filters to observe polarisation effects (5 minutes)
- Discussion: A simple explanation of polarisation (15 minutes)
- Demonstration: Polarisation of light, microwaves and radio waves (30 minutes)
- Demonstration: Polarisation of light by scattering (10 minutes)
- Student questions: Questions on polarisation (30 minutes)
- Discussion: A summary (5 minutes)
- Student activity: Aerials and polarisation (30 minutes)
- Student activity: Solutions may rotate polarisation (30 minutes)
Student experiment: Using polarising filters to observe polarisation effects
Provide each student with two Polaroid filters. Ask them to look through them at light sources (a lamp, the sky, (particularly at 90 ° to the Sun), etc.). Try one filter, then two. Rotate one relative to the other.
(It is helpful if the filters are rectangular rather than square, or marked in some way to help students keep track of the orientation.)
They should notice that one filter reduces the intensity of the light. A second can cut it out completely, if correctly oriented.
Discussion: A simple explanation of polarisation
Check that your students can recall the difference between transverse and longitudinal waves.
Point out that most wave properties are shared by both transverse and longitudinal waves, but there is one that distinguishes between the two – polarisation. Because this can only happen with transverse waves, it has given us useful information about the nature of waves.
Show this diagram; the blue wave is polarised in a vertical plane, and so can pass through a vertical slot. The red wave is polarised in the horizontal plane, and cannot pass through.
(Note that it is better to talk about
plane-polarised waves, rather than simply
polarised, as this will save confusion later.)
Discuss why longitudinal waves cannot be polarised.
Can students relate this to their observations with the Polaroid filters? Here is a simple explanation of how Polaroid filters work – use this if you think your students want a bit more explanation:
You will have to state that light (and other electromagnetic radiations) consists of oscillating electric and magnetic fields. Polaroid is a type of plastic; its molecules are long chains, oriented parallel to one another. There are electrons that are free to run up and down the chains
When the oscillating electric field is vertical, and the chains are vertical, the electrons are caused to move up and down with the same frequency. (The chains are like miniature aerials, absorbing the radiation.) At the same time, the electrons re-emit the radiation in all directions, and the result is that not much radiation passes straight through.
If the polymer chains are at right angles to the electric field, the electrons cannot move very far and thus do not absorb much energy from the wave, so it passes through. At any other angle, it is the component of the electric field perpendicular to the chains which passes through; this explains why the light dims as you rotate the filters.
Demonstration: Polarisation of light, microwaves and radio waves
Here you can show that light, microwaves and radio waves can all be polarised.
Demonstration: Polarisation of light by scattering
When light passes through a cloudy liquid, some is scattered. The scattered light is polarised.
Set this up in advance; show it briefly, and invite students to look at the transmitted and scattered light through polarising filters during the rest of the episode.
Use this diagram to help explain why scattered light is polarised.
Student questions: Questions on polarisation
It will help students if you explain that the length of an aerial is often one-quarter or one-half wavelength.
Make a selection of questions that you feel are relevant to your students.
Discussion: A summary
Summarise the ideas that you have been looking at: we know that electromagnetic waves are transverse because they can be polarised. Sound cannot be polarised, and so must be longitudinal. Emphasise that polarisation is good evidence for the wave nature of light; reflection and refraction can both be explained without recourse to the idea of waves. Later, students will see that interference and diffraction are both also characteristic of waves rather than particles.
Student activity: Aerials and polarisation
This could be a home experiment. It will not be possible for everyone to see every type of aerial but observations could be pooled and discussed.
In a radio, the ferriterod increases the magnetic field and so should be parallel to the magnetic field of the em radio wave(some specifications mention the alignment of aerials).
Student activity: Solutions may rotate polarisation
Polarimeters and the rotating effect of sugar; used in the sweet industry.
This could form the basis of an investigation.