Electromagnetic Wave
Light, Sound and Waves

Electromagnetic (EM) waves

Lesson for 16-19

Students will have a simple idea of electromagnetic radiation – that there are several types of radiation, similar to light, all of which travel at the same speed in empty space.

These episodes develop this picture. In particular, we start from the assertion that light is a transverse wave. How can we know? To explain this, we need to explore the idea of polarisation.

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Preparation for electromagnetic waves topic

Electromagnetic Wave
Light Sound and Waves | Electricity and Magnetism

Episode 312: Preparation for electromagnetic waves topic

Teaching Guidance for 16-19

Students will have a simple idea of electromagnetic radiation.

Main aims of this topic

Electromagnetic waves


Students will:

  • know some of the methods by which light can be polarised
  • be able to predict what will be seen or detected with a combination of filters and different orientations of detectors and transmitters
  • understand why only transverse waves can be polarised
  • appreciate that polarisation of light and microwaves tell us that electromagnetic waves are transverse
  • know the general properties of electromagnetic waves including the order of magnitude of the wavelength of each region and how each is detected
  • show the inverse square law experimentally and to be able to use it in calculations

Prior knowledge

Students should know the main sub-divisions of the electromagnetic spectrum.

They should also have studied some basic ideas about waves (how they are produced; reflection and refraction).

They should know the difference between transverse and longitudinal waves.

Where this leads

If students study these episodes before they have dealt with radioactivity, it will help them to understand the nature of gamma radiation, as well as the idea that the intensity of radiation diminishes as it spreads out.

Students will learn much more about light when they study diffraction and interference.

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Polarisation

Progressive Wave
Light, Sound and Waves | Electricity and Magnetism

Episode 313: Polarisation

Lesson for 16-19

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.

Lesson Summary

  • 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.

Episode 313-1: Polarisation of waves (Word, 313 KB)


Demonstration: Polarisation of light by scattering

Episode 313-2: Polarisation by scattering (Word, 37 KB)


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.

Episode 313-3: Polarisation of light by scattering (Word, 29 KB)


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.

Episode 313-4: Polarisation in practice (Word, 36 KB)


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).

Episode 313-5: Home experiments with radio and television signals (Word, 78 KB)


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.

Episode 313-6: Polarimetry (Word, 32 KB)


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Electromagnetic radiation

Electromagnetic Wave
Light, Sound and Waves | Electricity and Magnetism

Episode 314: Electromagnetic radiation

Lesson for 16-19

This episode extends students’ understanding of the nature of different types of electromagnetic radiation, and considers their shared nature.

Lesson Summary

  • Demonstration: Beyond the visible spectrum (10 minutes)
  • Student presentation: Regions of the electromagnetic spectrum (30 minutes)
  • Student questions: General aspects of the spectrum (30 minutes)

Demonstration: Beyond the visible spectrum

Set up a spectrum of visible light. To do this, shine a bright white light source through either a prism or a diffraction grating.

(Note that a prism is often used, but that a diffraction grating is much simpler and more reliable. At this stage, it is harder for students to understand how a diffraction grating works, but you can promise that they will soon know!)

Display the spectrum on a white screen.

Use a phototransistor or other infra-red detector to show that the spectrum extends beyond the red. Use fluorescent paper to show that it extends beyond the violet. (Alternatively, use a fluorescent marker pen, of the type used for security labelling.

(Be sure to check that these methods work before showing this to a class.)

It can help students to understand the continuous nature of the spectrum if you relate colours to temperatures. Any object emits infrared; heat it to 500  ° C and it glows dull red; heat it to 1 000  ° C and it glows white hot.

Student presentation: Regions of the electromagnetic spectrum

In advance of this episode, ask students to find out about a particular region of the electromagnetic spectrum. They should find out about:

  • wavelengths, frequencies and speeds
  • methods of production
  • methods of detection
  • uses

Note that the regions of the spectrum are not well-defined. You could include terahertz radiation, a part of the spectrum which is rapidly gaining importance.

An alternative approach would be to ask individuals to research the importance of different types of electromagnetic radiation in different spheres of interest – medicine, astronomy, communications, the historical development of physics, etc.

Ask students to present their findings in turn; provide a suitable blank table for them to record a summary.

This can be used as an OHT to show how the radio wave and microwave regions can be subdivided.

Episode 314-1: Signal bands for communications (Word, 38 KB)


Student questions: General aspects of the spectrum

These questions will help to focus students’ attention on the general features of the spectrum.

Episode 314-2: Charting the electromagnetic spectrum (Word, 47 KB)


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The inverse square law

Power
Light, Sound and Waves | Electricity and Magnetism

Episode 315: The inverse square law

Lesson for 16-19

This episode considers the ways in which the intensity of radiation decreases with distance from the source.

Lesson Summary

  • Demonstration and discussion: radiation spreading out, and an analogy (20 minutes)
  • Worked example: Thinking in proportions (5 minutes)
  • Student experiment: Estimating the power output of the sun (15 minutes)
  • Student questions: Comparing lamps (30 minutes)

Discussion and demonstrations: Radiation spreading out and an analogy

Set up three simple demonstrations:

  • Bright lamp and phototransistor or other light meter
  • Microwave source and detector
  • Gamma ray source and Geiger counter

Show that the intensity of each type of radiation decreases with distance from the source.

(If students have already studied radioactivity, they may be aware of the inverse square law for gamma radiation.)

Why does the intensity decrease with distance? Try to give two general reasons. (Absorption and spreading out.)

Radiation spreading out radially covers a bigger and bigger area, proportional to r2, so its intensity decreases as 1r2. It can help to think of a toast-buttering gun. The gun can butter a single slice of toast at a distance of 1 m. How can it butter four slices simultaneously? (Place them in a 2  ×  2 array at a distance of 2 m.) How thick will the butter be? (14 of original thickness.) Extend this to a 3  ×  3 array at 3 m, and so on.

Point out that the intensity of radiation can be measured in watts per square metre (W m-2).

Worked examples: Thinking in proportions

A cobalt-60 source gives a gamma dose rate of 160 μSv h-1 at 1.0 m away. At what distance will the dose rate be 40 μSv h-1?

Answer: If the intensity has gone down by a factor of 4, the distance away must have doubled to 2.0 m.

Or by

I = kd 2

I1I2 = (d2)2(d1)2

Student experiment: Estimating the power output of the sun

Students can make measurements to estimate the power output of the sun, making use of the inverse square law. Note that there is a good opportunity here to discuss the validity of the answer obtained.

Episode 315-1: Summer sun remembered (Word, 54 KB)


Student questions: Comparing lamps

Some calculations.

Episode 315-2: Comparing intensities for lamps (Word, 26 KB)


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