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AS Unit G492 - Understanding Processes

Chapter 6 - Wave Behaviour
Title

Description
Task 1a - Properties of Waves Part 1 Your teacher will show you examples of waves on a rope and a spring and remind you of a few key terms you need to know, along with a few new terms you need to integrate into your vocabulary!

Key terms: transverse, longitudinal, rest position, oscillation, amplitude, frequency, wavelength, peak, trough, compression, rarefaction.
Task 1b - Properties of Waves Part 2

Your teacher will show you a simulation of a transverse wave.  Try to use the terms you have just learned to describe the waves you see.  Your teacher will also explain what is meant by the term phase.

Teacher note: click here to load a copy from the local server.  Set to oscillate with an amplitude of 50, frequency of 25, no damping, no end.  Counting the waves passing a point in 10 seconds works well to illustrate idea of frequency.
Task 1c - Properties of Waves Part 3

Your teacher will show you an example of a sound wave propagating through the air.  What type of wave is this?  What features can you identify?

Teacher note: select wave type "sound" (2nd tab), expand the particle area with the green "+" sign, and select "particle view" with the radio buttons.  There is a local copy stored here, if the PhET website is inaccessible for any reason.
Task 1h - Wave definitions Write definitions for each of the terms on this list, in words you can understand.  You may wish to draw diagrams to help explain some terms.

wavelength, amplitude, frequency, period, phase, rest position, oscillation, transverse wave, longitudinal wave, polarisation
Task 2 - Loudspeaker and baffle Your teacher will show you the effect of placing a baffle in front of a loudspeaker.  The results may be surprising!  Your teacher will ask you to share your ideas, to try to explain what is happening!

Teacher note: it is worth looking at "demonstration activity 10D" on the Advancing Physics CD.
Task 3 - Hearing superposition Your teacher will show you a demonstration of one effect of wave superposition, using two loudspeakers and a signal generator.  Walk slowly in front of the loudspeakers - what do you notice?  Can you explain the differences you hear?

Your teacher will ask you to explain what is happening, using some of the same ideas you explored in the last task (Powerpoint slide of experimental set up here).  You may also like to look at the simulation below, which should help you visualise and investigate the interference of sound waves from two sources.

Note: there is a local copy of simulation here.

When you are happy you understand, try these questions (Q40S) which look at the mathematics leading to this effect.

Teacher note: it is worth looking at activity / experiment 30E on the Advancing Physics CD.  Also, questions Q50S may be useful for further analysis of the phenomenon.
Task 4a - Lloyd's mirror (microwaves) Your teacher will show you a demonstration of "Lloyd's Mirror" for microwaves.  Observe the effects of moving the mirror on the superposition of the waves.  When you are happy you understand, answer these questions (Q30S).
Task 4b - superposition and speed measurement Try these questions, which show how the superposition of reflected waves can be used to measure speed.
Task 5 - superposition of radio waves Try these questions (Q190S - "superposition of radio waves"), which look at the "fading" effect sometimes observed when radio waves are reflected from the ionosphere.
Task 6a - Young's Slits Your teacher will demonstrate what happens when coherent laser light passes through a narrow double-slit.  Observe the results carefully.  Can you explain what you see?  (You may want to think back to the results of the demonstration with the loudspeakers in Task 3!)

Listen carefully as your peers share their ideas - you will need to write an explanation of what you have seen using correct technical language.
Task 6b - Wave explanation of two slit diffraction (Huygens) These instructions guide you through drawing "Huygen's style" wave diagrams using a compass and graph paper, in order to explain two slit interference.
Task 6c - Wave explanation of two slit interference (path difference) Your teacher will explain the geometry behind the diffraction pattern you have observed, using [this presentation].  You may also wish to look at this applet, which shows how the interference pattern in Young's 2-slit experiment is formed in terms of the path difference of the waves from each slit.
Task 6d - Two source interference problems Try these homework problems (Q180S), which look at two source interference in a variety of situations.
Task 7a - Diffraction gratings Your teacher will show you this presentation, and explain the geometry which leads to the formation of bright and dark fringes through a diffraction grating.  Answer these questions (Q200S) about diffraction through gratings.
Task 7b - More diffraction problems These questions will give you a little more practice with the diffraction formula.
Task 7c - Measuring the wavelength of light (practical task) You are going to carry out a practical task to find the wavelength of light coming from 3 different LEDs, by measuring the positions of the fringes observed through a diffraction grating.  Follow the instructions on this powerpoint presentation to carry out the practical task.  Record your results on this sheet.
   
Task 8 - Wave superposition We have seen examples of what happens when two waves meet with differing relative phases.  Complete this activity (10W) to demonstrate your understanding of wave superposition.
Task 9 - Waves on a string Your teacher will demonstrate "standing waves" being formed on a string, using a signal generator and a mechanical oscillator.  Watch carefully and listen as your teacher explains how the standing waves are formed.  You may also find it useful to look at this simulation of a wave on a string.  When you are happy you understand, answer these questions (Q90S).
Task 10a - Waves in a pipe Your teacher will show you how standing waves can be set up in a pipe which is open at both ends, and explain how this can happen.  The waves produced can be observed on the oscilloscope; you should see a sharp increase in the amplitude as the fundamental frequency is reached, and then again at each of the harmonics.

Make a note of the frequencies at which standing waves occur.  Your teacher will show you how to use this data to calculate the speed of sound in air!
Task 10b - Waves in a pipe II  This simulation allows you to look at the fundamental frequency and harmonics of standing waves in a pipe.  See what effect the kind of pipe has (both ends open, both ends closed, or one end open).  Pay close attention to the arrangement of pressure nodes and pressure antinodes.  You will also find it useful to look at the arrangement of displacement nodes and displacement antinodes.

These summary notes give you the general rules used for calculating the wavelengths and frequencies of the harmonics in each type of pipe, with diagrams for the first few harmonics in each.  When you are happy you understand, try answering these questions (Q100S).

Teacher note - here are some starter questions on the same topic for the subsequent lesson.
Task 10c - Waves in a pipe III Starter questions - key terms used to describe standing waves (powerpoint presentation).

Suggested exam questions to try:

June 2002 Q1 (Section A, 3 marks)

Jan 2003 Q4 (Section A, 4 marks)

Jan 2001 Q9 (Section B, 10 marks)
Task 11a - Colours in thin films You will have seen colours formed in thin films, such as the films of oil which can sometimes be found floating on a puddle.  You are going to look at the colours formed by interference of light reflected by a soap film.  Follow the instructions on this sheet (Activity 210E) to set up the experiment.

Your teacher will show you this powerpoint presentation and help explain the process leading to the formation of the different coloured fringes, in terms of the path difference of the waves reflecting from the front and rear surface of the film, and hence their relative phase when the superposition occurs.  Think about the questions on this powerpoint presentation, which should help develop your understanding of why the fringes are formed.  Your teacher will give you a copy of these diagrams to stick into your exercise book.
Task 11b - Colours in thin films Starter questions for the subsequent lesson, in powerpoint format.
Task 12a - Phasors Your teacher will explain to you how phasors can be used to represent the instantaneous position of a wave in its cycle, and how these phasors can be used to find the resultant wave when two waves superpose.  You may find it useful to stick these diagrams in your exercise book.

When you are happy you understand, try these questions (Q30W).  It is also useful to see how the phasor translates to the more familiar "wave cycle".  Fetch a compass and some graph paper and try this activity (Q220S).
Task 12b - Phasors Try the following exam questions:

• Jan 2001 Q5 (Section A, 4 marks)

• June 2001 Q3 (Section A, 3 marks)

• Jan 2004 Q6 (Section A, 3 marks)