Interference
Quantum and Nuclear

Multiple contributions - Teaching approaches

Classroom Activity for 14-16

A Teaching Approach is both a source of advice and an activity that respects both the physics narrative and the teaching and learning issues for a topic.

The following set of resources is not an exhaustive selection, rather it seeks to exemplify. In general there are already many activities available online; you'll want to select from these wisely, and to assemble and evolve your own repertoire that is matched to the needs of your class and the equipment/resources to hand. We hope that the collection here will enable you to think about your own selection process, considering both the physics narrative and the topic-specific teaching and learning issues.

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Beats

Beats
Quantum and Nuclear | Light, Sound and Waves

Beats

Classroom Activity for 14-16

What the Activity is for

Generating and then hearing beats.

Hear, and perhaps see, how two contributions coming into and out of step can result in a third frequency that is not actually there.

What to Prepare

  • a police whistle
  • a computer linked to powered loudspeakers (do not try to run this experiment from your laptop speakers) and a data projector
  • Audacity or another sound processor

What Happens During this Activity

Choose a single frequency, say 1000 Hz, and generate five seconds worth of sound. Play this. Silence the channel.

Choose a new channel and generate five seconds of a new single frequency, say 1010 Hz. Play this. Silence this channel.

Now zoom in to both channels so that you can see about a 0.25 second of both sounds. Use the cursor to move through the channels, looking for a point where both signals are in step – that is they're going up and down together, albeit temporarily. Now scan through the channel until you meet a point where again they're going up and down together. Use either the time axes or the software facilities to measure the duration between these two points. Then ask how many times per second these frequencies come back into step, taking care to match the development of this point to the abilities of the class. Scan through the channel to confirm this prediction. Now use the idea of superposition to predict the frequency with which you'll get more and less sound if you play both channels.

Now play both channels. Listen very carefully: you'll hear a new frequency generated as a result of the first two. This frequency is 10 Hz. You can now repeat and extend the procedure but having chosen a new pair of generated frequencies, separated either by 10 Hz or a new different separation. Explore what you hear and, if you like, what you see. You might combine both channels using the software in order to show what is arriving at the ear.

As a finale, record the police whistle into the sound analyser, and you'll find two real frequencies, and one ghost frequency that is generated as a result of the contributions of these two. The ghost frequency depends on the difference between the real frequencies as this determines how often they are in and out of step.

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Polarisation explored

Interference
Quantum and Nuclear | Light, Sound and Waves

Polarisation explored

Classroom Activity for 14-16

What the Activity is for

Connecting polarisation to the everyday.

There are a number of standard laboratory demonstrations of polarisation, from those performed with 3 cm waves, through to those relying on pieces of Polaroid. These can be incorporated, but the focus of the activity here is to show how polarisation is a part of everyday life.

What to Prepare

  • some polarising filters
  • a pair of polarising sunglasses
  • some LCD displays
  • an analogue radio, with aerial, definitely FM and AM
  • a television aerial
  • a sheet of reflecting glass, a light sensor and a focused light source
  • a 3 cm transmitter–receiver pair

What Happens During this Activity

You might start with one of the standard demonstrations of polarisation, perhaps with a 3 cm transmitter– receiver pair. If either of these is rotated then the signal detected drops to nothing – it's really rather impressive. At this level we recommend not introducing the polarising grid unless this helps to explain what is going on with some of the later examples. So don't introduce it yourself, but have it to hand in case it's useful for the conversations that evolve.

After that it is perhaps best to go to a practical and challenging situation, which may have happened to the children already, of looking at LCD displays through polarising sunglasses. You might introduce this with a question, or a story. This will probably involve trying to read a GPS (satnav), a mobile phone screen, a digital camera screen or perhaps an e-reader screen. These might be more accessible contexts than the time-worn angler peering into the lake.

The point to make is that there is something funny going on that depends on angle. Here, making sense of the phenomenon depends on students already being familiar with the idea of polarisation – either through experiment, which you may have covered above, or through a theoretical approach, perhaps using the interactive provided in the physics narrative of this topic.

Images of rows of terraced houses, all with their television aerials aligned, are particularly useful to introduce the idea that other electromagnetic waves might also be polarised. Having a television aerial – actually in your hand – allows you to use your hands as well as your voice to bring alive the idea that the vibrations will be detected in one plane only. Much the same point can be made by using a radio with an aerial that can easily be angled. You'll need to practice in the laboratory where you are to carry out this demonstration in order to be sure that you can gain and lose a particular station by rotating the aerial. (The same station is often broadcast at different polarisations from different transmitters).

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Patterns in superposition

Interference
Quantum and Nuclear | Light, Sound and Waves

Patterns in superposition

Classroom Activity for 14-16

What the Activity is for

Hearing superposition, seeing a pattern evolve.

The idea here is to connect the somewhat abstract idea of superposition with a real physical experience on a human scale.

What to Prepare

  • a pair of loudspeakers
  • a signal generator

What Happens During this Activity

The loudspeakers should be connected to the same signal generator and facing a wide open space. If you have a large enough unobstructed space, this experiment can be very impressive. Some have even worked out of doors in a playground. It's important that you choose a suitably open space to avoid too many reflections. The loudspeakers should be several metres apart, even if in a laboratory. Choose a suitable frequency – several hundred hertz.

As the students move in a line parallel to the loudspeakers, so they will move through areas where there is constructive superposition and then into areas where there is destructive superposition. They will therefore hear a loud noise, followed by a somewhat quieter noise as they move along this line. Having a circulating queue is one technique that works well in the laboratory. Another is to use a higher frequency and simply to have the students move their heads approximately 1 metre from left to right, still parallel with a line drawn through the pair of loudspeakers. In the playground you might choose such a frequency that you are able to get a visual impression of the pattern stretching out over space by asking the students to choose a point to stand where they can hear lots and then, after a pause, where they can hear very little. If you're very lucky, you'll have an upstairs window from which to operate a digital camera in order to record these two states.

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Diffraction demonstrations

Interference
Quantum and Nuclear | Light, Sound and Waves

Diffraction demonstrations

Classroom Activity for 14-16

What the Activity is for

The purpose here is to show diffraction, to demonstrate that the light spreads. To do this you will need to practice and take care – the setup is not straightforward. However, it is not a situation where the physical can be replaced by images or by computer simulations. The sense of awe, wonder and disquiet engendered by physical immediacy is important.

What to Prepare

  • a bright and focused light source
  • a translucent screen
  • a dark plate with a 1 mm pinhole in the centre
  • a darkened laboratory
  • several objects as obstructions to be placed in the narrow beam (we suggest a dark sheet with holes of 2 millimetre, 3 millimetre and 1 cm in diameter; human hair; a ball bearing 3 millimetre in diameter; a round-headed pain; a set of three slits 3 mm, 0.3 mm and 0.03 mm across)

What Happens During this Activity

Place the lamp and pinhole at one end of the laboratory to throw a sharply defined beam of light across the room. Place the objects 3 metre from the lamp and then place the screen a further 2 m from the object. The students look at the screen towards the lamp: they stand behind the screen.

Introduce the objects to the students one at a time. Draw out expectations for the shadow that will be cast. Place the objects one at a time into the beam. To do this you'll need to mount the object to make it easy to hold, say on a piece of plate glass or perspex – whatever it is it needs to be transparent. The idea here is to replace the simple rule that light travels in straight lines, so producing sharp shadows, with a more subtle interpretation that draws on the idea of this topic. The patterns are truly awesome. They are one of the strongest pieces of evidence that students will see to suggest that one needs a proper model of radiations and radiating to understand the phenomena. Here they have seen all the examples, but you might well draw connections to hearing things, where sound often travels around corners. This is less surprising, so perhaps not such a good place to start, but the connections between the behaviours of light and sound need to be made.

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A model of a lens

Interference
Quantum and Nuclear | Light, Sound and Waves

A model of a lens

Classroom Activity for 14-16

What the Activity is for

Making a lens with string.

This is set up as a bench top experiment, but really it's a piece of practical mathematics. If run well, it can drive home the function of do like me, but later, trip time, and therefore geometry in the design and explanation of lens action.

What to Prepare

  • several trip time rulers (these will be different colour rulers, marked out in nanoseconds – you might choose yellow to mark out the times to cover respective distances in glass, and green to mark out those in air)
  • a large piece of paper, suited to the scale of the rulers
  • an optical smoke box, for demonstrating lens action
  • perhaps 3 cm waves and wax lenses

What Happens During this Activity

Start out by marking out a source and a detector on the large piece of paper. Explain that the purpose of the very special optical device we are going to design is to ensure that by whatever path the light travels from source to detector, the contributions from different paths will be in step at the detector. You're likely to have to revise the idea of what it means to be in step – simple enactment like hands going up and down together can be evocative and interactive here. Link this to the instruction to do like me, but later, and draw out the importance of trip time.

Now sketch out some possible paths for the light as it passes from the source to the detector. The argument then needs to be developed as follows.

The one that takes the least time, directly from source to detector, is the one that presents most problems. Any other path will take longer, so somehow we need to increase the length of time between the source and the detector for this path. Any suggestions – can anybody think how we can make light take longer to get from the source to the detector? Draw out the idea that the trip time for each centimetre, millimetre, or metre is greater in glass than in air. So inserting extra glass in this path will increase the trip time.

Now focus on paths defined by waypoints at the top and the bottom of the paper. These will necessarily take a very long time, because there is a great distance to be covered. For these paths, therefore, you want no glass in the way and the light to be travelling through the air for the entire path.

Paths that have waypoints between the extremities and the centre will require the insertion of the lengths of glass somewhere between zero and the length inserted for the direct path, in order to compensate for the shorter distances that the light has to cover in traversing the more direct parts.

Now reinforce this hand-waving argument, by using the rulers to measure the lengths of time and make them equal. In this way it's possible to work out exactly how much glass has to be inserted. Joining up the required lengths of glass, denoted by the appropriately coloured rulers, with a smooth curve will reveal a focusing lens.

To go with this you'll want a good lens demonstration, such as that provided by a smoke box, which you can find on practicalphysics.org. You might also have access to wax lenses for 3 cm waves. These can be impressive. And emphatically make the point that the mechanism works across many different frequencies.

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Modelling a mirror

Interference
Quantum and Nuclear | Light, Sound and Waves

Modelling a mirror

Classroom Activity for 14-16

What the Activity is for

Making a mirror with string.

This is set up as a bench top experiment, but really it's a piece of practical mathematics. If run well, it can drive home the function of do like me, but later, trip time, and therefore geometry in the design and explanation of mirror action.

What to Prepare

  • several long trip time rulers, marked out in nanoseconds, foldable
  • a large piece of paper, suited to the scale of the rulers
  • a demonstration parabolic mirror, perhaps in a smoke box

What Happens During this Activity

Start by marking out a set of points along one side of the paper as the start of a set of paths, representing a beam. Make the point that the vibrations in the beam crossing that edge of the paper could all be in step. Somewhere towards the centre of the paper, mark a detector. Now the challenge is to design a mirror so that all the contributions reach the detector still going up and down together – that is, in step.

Suppose you have a plain mirror at the back of the paper. The path from the centre of the paper, passing by the detector, bouncing off the mirror, and back to the detector, will cover the shortest distance. The path from the edge of the paper, passing straight back to the mirror, bouncing off the mirror, and then angling down to the detector will travel the greatest distance.

Start by developing the outlines of the argument.

Paths between these two extremes will travel distances somewhere between these two values. Since the light is travelling through air at all times, the trip times will depend only on the distances. The speed is constant. So the only way to make the trip times equal is to reduce the distances of the angled paths, coming from the extremities. One way to do this is to move the mirror at these extremities closer to the initial points, defining the start of the beam. The farther away from the centre line of the mirror – that is the line joining the mirror and the detector – the greater this shortening will need to be.

Now produce the rulers. Start with one in the middle, placing one end where the beam starts and folding the ruler where the path bounces off the mirror. Read off the trip time.

Use a new ruler, starting at another marked point farther away from the centre line. You know how long the trip time has to be – the same as the first one – so bend the ruler at an appropriate point. Repeat for all the remaining points that define the paths to be explored. Join the points where the rulers bend, and you'll have a parabolic mirror.

To go with this you'll want a good mirror demonstration, such as that provided by a smoke box, which you can find on practicalphysics.org. You might also have access to mirrors for 3 cm waves or for infrared: these can be impressive and emphatically make the point that the mechanism works across many different frequencies.

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Diagnostic questions on superposition

Interference
Quantum and Nuclear | Light, Sound and Waves

Diagnostic questions on superposition

Diagnostic Questions for 14-16

What the Activity is for

Diagnostic questions exploring the diffraction, interference and polarisation of waves.

The diagnostic questions can be used for two main purposes:

  • To encourage students to talk about and think through their understandings of the diffraction, interference and polarisation of waves.
  • To provide the teacher with formative assessment information about the students' understandings of the diffraction, interference and polarisation of waves.

What to Prepare

  • printed copies of questions on diffraction, interference and polarisation (see below)

What Happens During this Activity

We'd suggest getting the students to work in pairs on these questions, encouraging them to talk through their ideas with each other. Collect responses from all of the pairs and discuss in a whole-class plenary.

Alternatively, the questions might be set for homework prior to the lesson, so that you have time to read through the responses.

Question 1: The spreading of a wave through a gap in a barrier is called:

  1. interference.
  2. superposition.
  3. dispersion.
  4. diffraction.

Answer 4.

Question 2: Light passing through a small pinhole does not make a shadow with a distinct, sharp edge because of:

  1. refraction.
  2. diffraction.
  3. polarisation.
  4. interference.

Answer 2.

Question 3: A train of water waves travels from an area of deep to shallow water, crossing the boundary at right angles. Which of the following statements is/are correct:

  1. The water waves change direction as they meet the boundary.
  2. The water waves do not change direction as they meet the boundary.
  3. The water waves speed up as they meet the boundary.
  4. The water waves slow down as they meet the boundary.
  1. 1 only.
  2. 2 only.
  3. 1 and 3.
  4. 2 and 4.

Answer 4.

Question 4: Which of the following kinds of waves can show interference effects?

  1. sound
  2. water
  3. infrared
  1. 1 only.
  2. 1 and 3 only.
  3. 2 only.
  4. 1, 2 and 3.

Answer 4.

Question 5: Which of the following settings in a ripple tank will produce the most prominent diffraction pattern of a water wave of wavelength of 2 cm?

  1. The wave passes through a gap of 20 cm.
  2. The wave passes through a gap of 10 cm.
  3. The wave passes through a gap of 4 cm.
  4. The wave passes through a gap of 2 cm.

Answer 4. For maximum spreading to occur the size of gap needs to be the same as or smaller than the wavelength.

Question 6: The polarisation of light is best explained by thinking about light as being:

  1. a longitudinal wave.
  2. a transverse wave.
  3. a transverse wave with components in different planes.
  4. a stream of energy photons.

Answer 3.

Question 7: Light is passed through a polarising filter with its transmission axis in the horizontal plane. It then passes through a second filter with its transmission axis in the vertical plane. After passing through both filters the light will be:

  1. polarised.
  2. unpolarised.
  3. entirely blocked.
  4. returned to its original state.

Answer 3.

Resources

Download the support sheet / student worksheet for this activity.

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