HSC Component: 'The World Communicates'

some notes by Joe Wolfe
School of Physics
The University of New South Wales
Sydney 2052 Australia

These notes were prepared for a workshop for high school physics teachers in New South Wales, Australia, where a new senior high school physics syllabus (the Higher School Certificate or HSC) is to be introduced. Quotation marks in this document usually indicate quotation from that syllabus. (Physics Stage 6 Syllabus, copyright Board of Studies NSW, 1999.)


Suggestions for examples and strategies for teaching the syllabus headlines

Using sound for a variety of wave examples. 'Sound waves can be used to illustrate many of the properties of waves that are utilised in communication technologies.'

Superposition in air

Radiation and travelling waves using sound

  • If you have a spectrum analysis package for your sound card (or a digital CRO with this feature), then each team could use a different frequency, not harmonically related, and the teacher could use industrial grade hearing protection and aspirin.

    'Model the effect of different materials on the reflection and absorption of sound'

    Speed of sound in air Speed of sound in other media. Time of flight methods are usually complicated by reflections.
    A stroke rod is a long (> 1 m) cylinder of a metal (Al is good) in which standing waves are excited by holding at a node (start at the centre) and rubbing at a non-node with a cloth containing weight-lifters rosin. The harmonic series is possible. Measure the frequency using a CRO or a musical ear. This musical instrument is acoustically like the bore of a flute, but uses a longitudinal wave in Al instead of air for the standing wave. (The SSO has a set of these instruments, but commercial Al rod is almost as good for our purposes.)

    Waves in slinky springs, water waves, ropes, strings. Displacement-time graphs

    To reduce friction, suspend a slinky spring from a rod or from the ceiling by many threads of equal length threads (the longer the better for transverse waves in a nearly linear medium). This is a good way to allow a wave under low tension - and therefore speed: slow enough to see in detail. This method also shows reflection at a free end (or at least a big change in impedance) more easily than most media. The tension is controled by a thread at the end (~ free end for transverse wave), or by holding it firmly in a hand (~ fixed end). Slinkies can do transverse and longitudinal waves.

    For water waves, if you have ripple tanks you will probably have gadgets that came with them, including vibrating bars for making beams of plane waves, and an instruction manual. In most cases, reflections are complicating (but interesting) factors. Remember that water waves have non-linear superposition when amplitude is not << depth - much loved by surfers, but a complication for superposition.

    For ropes, rubber ropes (we use the flexible hose that your chemistry lab may use for the Bunsen burners) have some advantages. One is that the length can be used to control the tension for reproducibility. Remember that ropes become non-linear media when the displacement changes the tension (another advantage for rubber hose). Comparing stretched ropes (observable motion at low tension and large mass) with musical string instruments (which do the same thing too fast to see but fast enough to hear) is a useful teaching exercise.
    Musical string instruments give convenient examples of standing waves in stretched strings. These are very good for v = f*lamda (and for the effect of tension and string mass per unit length on v). The electric guitar has a few ~ velocity transducers (pick-ups) built-in, and is a familiar context for many students. A violin or bass bow is useful for exciting quasi continuous standing waves. Harmonics are easier with a bow. (Get fibreglass ones for long life.) (For more on harmonics, see our page strings).

    A stroboscope, running at a frequency of n/m times that of the vibration (n and m integers) can be used to 'freeze' the motion of a periodic vibration e.g. a string in a combination of modes or a drum head in a single mode*. If n = 1, one image per cycle is seen. For neurological reasons it is unwise to look at a strobe flashing at frequencies between 2 and 10 Hz.
    * With the approximate exceptions of timpani and tabla, drum head mode frequencies are not at rational fractions. However, the second lowest mode often lasts lower than the others.

    "present and analyse displacement-time graphs for longitudinal and transverse wave motion"
    Direct experimental measurements are tricky, although one can sketch qualitative observations of travelling and standing waves in a slinky or a rubber hose. The envelope of standing waves in guitar strings can be seen and sketched. Velocity sensors (electric guitar pickups) and pressure sensors (microphones) are much easier for measurements. Observing that the velocity has no DC component (the string doesn't fly away and there is no wind), students can integrate the signal graphically, or you can integrate it on an RC circuit to display displacement (from pressure to displacement is two integrations).

    Experiments with mobile phones
    Turn on, wrap in Al foil and phone its number. Unwrap gradually. Demonstrates that the good conductor reflects EM waves. More importantly, it will also stop the phone ringing during class.

    Light rays and geometrical optics
    Laser diodes are a cheap way of getting a beam with low divergence (buy directly from an electronics or hobby store for several $ including power supply: just add a battery. Or else buy assembled in laser pointers). Can use to demonstrate ray representation of fibre optics. They're also fun toys for a while and may seem more interesting than light boxes (lamp source and cylindrical lens, gives a vertical plane of light). For plotting on paper, light boxes are immediately usable. A laser can also make a vertical plane of light by passing it through a cylindrical lens (axis horizontal). A suitable lens is a short section of plexiglass rod. If you have light boxes, you probably also have a kit that goes with them: hemi-cylinders for Snell's law, 2D lenses for making optical instruments, cylindrical mirrors et hopefully cetera. (By the way, cylinders of perspex make good model raindrops to show how rainbows form.)

    Some notes about a guided investigation
    This was one of the FAQ's and so I felt obliged to answer it: How to avoid the recipe lab exercise, but still guide an investigation? I have almost zero experience in teaching high school students. Mosr readers of this document will have formal qualifications and extensive experience in doing just that. Further, the labs that I have tried to run along the lines tried here had relatively small numbers of students and taught different material. So it is with temerity that I offer some suggestions about trying to make an investigation less recipe based. I shall use Snell's law (not widely considered exciting) as an example. For me, the aim of teaching Snell's law is less important than teaching the method of careful observation, improvement of measurement technique, generalisation and testing.

    Some demonstrations to arouse interest, e.g. dismantle camera, use telescope/ microscope, set fire to paper using lens.
    Some initial contextual question(s): Where do you find lenses? How does a lens work? Where would we be without them? Who invented them? Who invented ?
    Does light travel in straight lines? When?
    When it doesn't, what happens? Explore and generalise qualitatively. Some questions: why does the water look shallower? why does the surface of the water look silvery and strange to swimmers looking up?
    What are the consequences for vision? For natural phenomena? For optical instruments? (higher performance; mirages, distorted sun, green flash; spectacles, cameras, telescopes)
    Paper work. What sort of relations might be involved? If you draw a diagram of wave fronts incident on a surface, what would light bending at an interface look like?
    How can you obtain some relevant quantitative data using the gear we have here?
    What makes the arrangement complicated? Are there ways you can reduce/eliminate the complications or number of things to measure?
    What will be the measurement errors? In particular, what is the largest measurement error (protractor? ruler? beam centre estimation?) and is there any way of making it smaller?
    Are there some errors that can't be reduced and thus impose a limit to the precision of your experiment? If so, can you reduce the other errors to this level?
    Can you get a quantitative rule out of this? If different teams find different rules, what are the cases where they disagree? Can you test them to see which is supported experimentally? (If they don't disagree, can they be proved to be identical?
    Suppose that you don't get Snell's law, or that you get a silly value for n, what then? Could the text books be wrong? Could we have made an error in interpretation, or a systematic error? How badly does Snell's law fit our data? How does the difference compare with the error?
    Application: back to some of the examples, draw qualitative diagrams to explain operation. How does a lens or a mirage work? Do a quantitative ray diagram?

    Some useful gear that is either or both cheap and readily available.

    A few web resources:

  • Physics in a suitcase - "Portable lecture demonstration kits in light and optics". Some are suitable for investigative lab work.
  • "VLAB" Download some simulations, including reflection & refraction, 'optics bench' with components, interference, superposition, wave motion, waveguides.
  • Spectrum_analyzers A range of spectrum analysers
  • Goldwave Another spectrum analyser
  • Light and Matter A downloadable text book.
  • The American Physical Society's educational links
  • Acoustics of music Our site on musical acoustics has quite a bit about standing waves and the operation of musical instruments. It has serious research results, but also has a lot of introductory material aimed at high school level.
  • More sites arelinked to the UNSW School of Physics' new HSC site: HSC resources, which will be updated from time to time. It has notes from us, lists of some of the sources of our gear, and of course a bunch of links. This site will grow, and will host a discussion group. You are invited to suggest things that we might add to it, to send material for it, or URLs that you think should be added.

    Some notes about the physics of speech.

    Opinions expressed in these notes are mine and do not necessarily reflect the policy of the University of New South Wales or of the School of Physics.

    Joe Wolfe / J.Wolfe@unsw.edu.au/ 61-2-9385 4954 (UT + 10, +11 Oct-Mar)

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