                  8.2 Worksheets    CONTENTS

WAVES PRE-TEST:

Answer T (True) or F (False) to each of the following:

1.      Current technologies associated with information transfer use waves of one kind or another.

2.      Light and sound are both examples of types of waves.

3.      A light wave can travel through a vacuum but a sound wave cannot.

4.      The wavelength of a wave is defined as the distance between two consecutive identical points on the wave eg between two crests.

5.      Sound waves travel faster through the water than they do through air.

6.      Light travels about 300 000 km in a single second through air.

7.      An echo is simply a reflected sound wave.

8.      Thick woolen curtains are better absorbers of sound than a concrete wall.

9.      When two waves travel through the same material at the same time, the total displacement of the material at any point is the algebraic sum of the individual displacements at that point.

10.  Gamma rays and radio waves are both part of the electromagnetic spectrum.

11.  Short wave radio waves are able to travel over the observable horizon because they are reflected off the ionosphere.

12.  Microwaves are used to communicate with satellites due partly to their ability to pass through the ionosphere.

13.  Infrared radiation can be used to guide missiles and to link computers together in networks.

14.  Light waves can carry information huge distances through optical fibres with little, if any, loss.

15.  Lasers are very useful tools in the communications industry.

16.  One advantage of an FM signal over an AM one, is that it is not susceptible to electrical interference.

17.  Doppler radar can determine the velocity of an approaching target.

18.  Digital Versatile Discs (DVD’s) store data in digital form.

19.  Light coming into our eyes from the environment is usually partially polarized.

20.  Many communications technologies use applications of the reflection and refraction of electromagnetic waves.

WORKSHEET No.1:

1.      A wave travelling on a string has a wavelength of 0.10 m and a frequency of 7 Hz.  Calculate the speed of the wave.

2.      A sound wave travelling in water at 1440 ms-1 has a wavelength of 0.5 m.  Determine the frequency of the wave.

3.      An electromagnetic wave moving through free space at 3 x 108 m/s has a frequency of 4.62 x 1014 Hz.  Find the wavelength of this wave and express it in nanometres.

4.      A water wave is moving across the surface of a lake in an easterly direction.  The wave has a wavelength of 2 m and a frequency of 2.5 cycles/s.  Draw a diagram of this situation, looking down from above the lake, showing 5 wavefronts.  Label the wavelength and show the direction of propagation by using a ray.

5.      A sound wave is moving through air.  The wave has a wavelength of 0.65 m and a frequency of 512 Hz.  Draw a diagram of this situation, showing 5 wavefronts moving due north.  Label the wavelength and show the direction of propagation by using a ray.

6.      A light wave moving due east through the air at 3 x 108 m/s has a frequency of 5.55 x 1014 Hz.  Draw a diagram showing 4 wavefronts, the wavelength and propagation direction.

7.      The following two graphs represent data from the same wave travelling due west through a particular medium.  From the graphs determine: (a) the wavelength; (b) the amplitude; (c) the period; (d) the frequency and (e) the velocity of the wave. 8.      Give one example of each of the following: (a) a one dimensional transverse wave; (b) a two dimensional transverse wave and (c) a three dimensional longitudinal wave.

WORKSHEET No.2:

1        Calculate the absolute refractive index for a clear plastic material, if the velocity of light in the plastic is 2.5 x 108 ms-1. (1.2)

2        A ray of light in air is incident at an angle of 40.8o on the surface of the same plastic material used in Q.1.  Determine the angle of refraction in the plastic.  (33o)

3        A ray of light passes from kerosene to glass.  The angle of incidence of the light is 45.2o and the relative refractive index from kerosene to glass is 1.08.  Calculate the angle of refraction in the glass.  (41o)

4        Using relevant information from Q.3, calculate the absolute index of refraction of kerosene if the absolute index of refraction of glass is 1.5.  (1.39)

5        A ray of light passes from air into a glass prism at an angle of incidence of 35o.  If the angle of refraction in the glass is 23.7o, what is the speed of the light in the glass?  (2.1 x 108 ms-1)

6        The absolute refractive index of water is 4/3 and that of glass is 3/2.  Find the relative refractive index for light traveling from water to glass.  (9/8)

7        The critical angle for diamond is 24o.  Determine the refractive index of diamond.  (2.46)

8        Fused silica has a refractive index of 1.46.  Calculate its critical angle.  (43.2o)  Find the subsequent paths of rays of light incident internally on the surface of fused silica at angles of incidence of:

(a)   35o

(b)   65o

9        Describe an application of total internal reflection used in the communications industry.

10    For yellow light, the refractive index of glass is 1.6 and the refractive index of water is 1.2.  Which of the following statements is correct?

(a)   The wavelength of yellow light in glass is longer than the wavelength of yellow light in water.

(b)   For the same angle of incidence, yellow light is refracted more by water than glass.

(c)    Total internal reflection cannot occur when yellow light travels from water to glass.

(d)   Light travels faster in glass than in water.

OSCILLOSCOPE PRACTICAL WORKSHEET

1        Students write a report to present the results of each of the sections below.

2        Explain briefly how an oscilloscope works and what it is used for.  Include the meaning of the y and x axes (wave amplitude & period respectively).

3        Check that the oscilloscope time scale is set to “normal” rather than 5X magnification.  These buttons are found in the time scale control area of the oscilloscope.

4        Attach the oscilloscope probe to a microphone and demonstrate each of the following.  Note that the oscilloscope settings are suggestions only and will obviously depend on the type of microphone used.

¨      A wave of single frequency (use a tuning fork) produces a sine wave on the oscilloscope.  Initial oscilloscope settings – Time = 0.5 to 2.0 ms/div & Volts = 50 mV/div and adjust from there.

¨      Whistling a single frequency produces a sine wave trace.

¨      Singing a song, speaking and clicking ones fingers together produce more complex waveforms.

¨      The louder the sound, the higher the amplitude (vertical axis value) of the waveform.

¨      The higher the pitch of the sound, the closer together the waves seem to pack along the x-axis.  That is, the higher the pitch, the lower the period and the higher the frequency of the waveform.

¨      Measure the period of the signal from a 320 Hz tuning fork and thereby calculate the frequency.  You should be able to get fairly close to 320Hz.  Use time = 1 or 2 ms/div (with variable sensitivity set to maximum) and volts = 10 mV/div.

¨      Demonstrate the production of beats using two identical tuning forks, with a small amount of blue tack added to one prong of one of the forks to produce a slightly lower frequency.  Show what the waveform looks like by using the microphone to receive the signal.

5        Attach the probe to the low output terminals of an audio frequency signal generator.  Select sine wave & microphone on.  Demonstrate what a signal generator does.  Measure the frequency of a particular signal using the oscilloscope.  (eg Set the signal generator on 400 Hz & measure the period of the wave produced.  From this calculate the frequency.  Use time = 1 ms/div.)  Also demonstrate once more that as you increase the frequency of the input signal, the horizontal spacing between crests on the screen decreases – that is, the period of the wave decreases.

6        Attach the wave generator to input A of the oscilloscope and the AC terminals of a power pack set to 2 or 4 volts to input B.  Display both signals on the oscilloscope at once using the A & B display buttons.  Adjust the signal from the signal generator until its frequency is almost identical to the signal from the power pack.  Then display the two waveforms added together using the A+B button.  Observe the oscilloscope trace vary with time in the usual beat fashion.  (Settings: 2V AC power pack signal – Volts = 2 V/div & time = 5 ms/div; 50-60 Hz approx wave generator signal – Volts = 50 mV/div & time = 5 ms/div.)

7        OPTIONAL: Attach an oscilloscope probe to the exposed wires of an earphone jack that is receiving an AM signal from a radio – preferably receiving speech rather than music.  Get a student to check that the wave pattern on the oscilloscope seems to match the voice pattern from the earphones.  Point out the carrier wave and the amplitude modulated signal on the oscilloscope.

This prac should take 1 to 1.5 periods but addresses several sections of the syllabus: 8.2.1 Column 3 dotpoint 3 and 8.2.2 Column 3 dotpoints 1 & 2.

The following is an example Practical Report write-up.  The Prac itself is NO LONGER required by the syllabus but can still be used to provide students with an example of how to write Physics Practical Reports.  After performing the practical, students are given the Aim, Method & Results sections as shown below & are asked to write the Discussion & Conclusion.  Alternatively, if time is short, the Prac does not even have to be done.  The Teacher can simply give out the Aim, Method & Results sections, discuss the results with the students & demonstrate how a discussion & conclusion could be written.

### SOUND PRACTICAL

DATE DONE: 15/3/02

AIM:

To investigate the extent to which various materials reflect and absorb sound.

METHOD:

1.      Data on the reflection and absorption properties of different materials were examined to enable the selection of appropriate materials for this investigation.  Data used was from Harding, J. et al (1996). “Physics Concepts & Applications VCE Units 3 & 4" Melbourne: Macmillan Education Australia Pty Ltd

2.      Using the experimental set-up shown in Diagram No.1, a sound intensity meter was used to measure the intensity of sound of constant frequency, produced by an audio frequency generator, after the sound had travelled the length of cardboard tube A.  The end of Tube A in contact with the audio generator was positioned so as to completely cover the speaker outlet in the back of the generator.

3.      The sound intensity was then measured at the end of tube B, after the sound had been reflected from plane surfaces composed of various different materials.  The plane surfaces were positioned so as to cover the opening at the intersection of Tube A with Tube B, as shown in the diagram.

4.      The results were tabulated and analyzed qualitatively to determine the extent to which different materials reflect sound.

5.      Using the experimental set-up shown in Diagram No.2, the intensity of sound of constant frequency was measured at a set distance from the audio generator, when no absorbing material was covering the speaker outlet.

6.      Styrofoam cups containing various absorptive materials were then placed in turn to completely cover the speaker outlet.  For each material the sound intensity was measured at the set distance from the generator.

7.      The results were tabulated and analyzed qualitatively to determine the extent to which different materials absorb sound.  RESULTS:

Reflection of Sound from Various Materials:

Sound intensity at end of tube A (ie before reflection), I1 = 110 dB

Background sound intensity with audio generator off was very much lower than 110 dB and was neglected in this experiment.

###### TABLE No.1: Sound Intensity Values – Reflection
 TYPE OF MATERIAL REFLECTED SOUND INTENSITY, I2 (dB) Difference I1 – I2 (dB) Foam Rubber 104 6 Low pile Carpet 105 5 Felt 104 6 Human Skin/Flesh 105 5 Leather 107 3 Styrofoam 107 3 Thick pile of paper 108 2 Thick cardboard 108 2 Hard plastic 109 1 Wood 110 0 Glass 110 0

* Note: Sound intensity at end of Tube B when no material was placed over opening at intersection of Tubes A and B = 104 dB.  (It is worth considering why you get any sound intensity at all in Tube B when there seems to be nothing to reflect sound into the tube.  Think about boundaries.  The air confined inside Tube A is of slightly different density to that outside Tube A.  Thus, there is a boundary at the open end of Tube A and reflection, transmission and absorption all occur at this boundary.)

Absorption of Sound by Various Materials:

Sound intensity at set distance from audio generator when no absorptive material was covering speaker outlet, I1 = 88 dB

Background sound intensity with audio generator off was very much lower than 88 dB and was neglected in this experiment.

###### TABLE No.2: Sound Intensity Values – Absorption
 TYPE OF MATERIAL SOUND INTENSITY AFTER ABSORPTION, I2 (dB) Difference I1 – I2 (dB) Foam Rubber 68 20 Paper Towels (crumpled into balls) 70 18 Styrofoam* 75 13 Bubble Plastic (as used in packaging) 75 13 Woodchips (fine)* 70 18

*Note: To avoid spillage several tissues were used to pack the end of cups containing these materials.

DISCUSSION:

To assist you in the writing of your discussion, consider the following questions:

¨      What do the data in Table No.1 suggest about the sound reflection properties of the materials used?  Which is the best reflector?  Which is the worst reflector?

¨      Is this what you expected?  If not, why not?

¨      Were there any anomalies in the data?  If so, how do you account for these?

¨      How do you explain the result mentioned in the “Note” at the bottom of Table No.1?

¨      What were some possible sources of error in this experiment?

¨      What suggestions can you make to improve the precision and accuracy of this experiment?

¨      Discuss the data in Table No.2 in a similar manner.

CONCLUSION:

When you have written a brief Discussion section, complete this report by writing an appropriate Conclusion, under that heading.

## Inverse Square Law for Light Experiment

Instructions:

Aim: To demonstrate that the relationship between the intensity of light and the distance from the light source is an inverse square relationship.

Method: Use a high wattage incandescent light globe as a light source.  Take measurements of the light intensity at set distances away from the source using either a digital data logger connected to a light intensity sensor or an analogue light intensity meter.  Use a tape measure to mark out the distances.  Draw a diagram of the set up – so leave space for this.  Mention that you will analyse the results by drawing a graph of light intensity versus (1/ distance squared).  Remember – past tense and no personal pronouns.

Results: Draw up the table as shown below.  You can complete the second column.

Table No.1: Light Intensity versus distance values

 Distance (m) 1/distance squared (m-2) Light Intensity (lux) 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Use graph paper AND draw the graph manually NOT in a spreadsheet on a computer.  Take care to use as large a scale as you can.  You should try to fill as much of the graph paper as possible.  All graphs have a title at the top and have both axes labelled with the name of the quantity being graphed and the unit of that quantity.  All data points should be visible after you have drawn your graph, so mark them clearly, usually with a small cross.  Remember you are plotting light intensity versus (1/distance squared) NOT light intensity versus distance.  Think carefully before plotting your points.  Remember also that you are looking for a straight line graph – but the best you can hope for in most experiments is a straight “line of best fit”

If you get a straight line of best fit when you plot your graph, state clearly in your results section what that means – that the two variables plotted are proportional to one another.  Therefore, light intensity is proportional to the inverse square of the distance from the source.

Discussion: Discuss the meaning of your results.  Did you get what you expected?  If so state what that was.  If not, attempt to explain the results that you obtained.  In either case you should comment on the major sources of error in this experiment & how the experiment could be improved.  Note – avoid using terms like “human error” when discussing errors in experiments.  Be specific.  If you mean that it was difficult to hold the light sensor steady when recording values of light intensity, then say that and explain how that may have adversely affected the results.

1.      Identify the independent and dependent variables in this investigation.

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2.      State which physical factors you have tried to keep constant and explain why you have tried to do this.

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3.      Identify the physical factors that are most likely to have adversely affected the accuracy of your results and explain how each factor affected the accuracy.

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4.      Describe how you would improve the reliability of your results.

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5.      Comment on the validity of your investigation.

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6.      Identify the relationship between the independent and dependent variables in this investigation and provide evidence to support your view.

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Conclusion – State clearly and very briefly what you can conclude from this experiment.

### LIGHT PRACTICAL

AIM:

To study the reflection and refraction of an electromagnetic wave (light), using the standard school laboratory Optics Kit.

METHOD:

1        Using the light source and slit plate, observe the path of light rays and construct both a ray diagram and a wavefront diagram to indicate the direction of travel of the light.

2        Present information using ray diagrams to show the path of waves reflected from:

¨      A plane surface

¨      A concave surface

¨      A convex surface

In each diagram clearly label the angle of incidence (i), the angle of reflection (r) and the normal to the surface.

3        Tabulate the data collected in part 2 above and comment on any similarity between the sizes of the angles of incidence and reflection.

4        Present information using ray diagrams to show the path of light as it moves from air into either a glass or perspex prism for five different angles of incidence ranging from 15o to 60o.  Clearly label the angle of incidence (i), the angle of refraction (r) and the normal to the surface.

5        Tabulate the data collected in part 4 above, using separate columns for i, r, sin i, sin r and sin i/sin r.

6        Plot graphs (on graph paper) of:

¨      Angle of incidence (vertical axis) v’s angle of refraction (horizontal axis)

¨      Sin i (vertical axis) v’s sin r (horizontal axis)

7        For a given pair of media, the ratio sin i/sin r can be shown to be a constant called the relative refractive index of the material for light passing from medium 1 into medium 2.  For light passing from air to glass (or perspex) the theoretical value of the relative refractive index is 1.5.  From your sin i v’s sin r graph, calculate an experimental value for this relative refractive index.  Compare the result with the theoretical value and propose an explanation for any difference.

8        Using the semi-circular perspex prism, observe the total internal reflection of light.  Draw a ray diagram to record this observation and clearly label the critical angle (ic).

9        Given that the theoretical value for ic is given by: where
m1 = absolute refractive index of medium 1 (1.5 for glass & perspex), calculate the theoretical value for the critical angle of glass or perspex and compare this with the experimentally obtained value.

REFRACTIVE INDEX PRACTICAL

AIM: To determine the refractive index of a material.

METHOD:

1.      Using a standard school laboratory Optics Kit, set up a ray box with a single slit aperture.  Connect it to a DC power supply set to 12V.

2.      Place a rectangular perspex prism in the middle of a white sheet of A4 paper so that its long edge is parallel to the long edge of the paper.

3.      Trace carefully around the prism to mark its position on the paper.

4.      Remove the prism and at ONE point somewhere near the centre of the left hand side of the prism draw the normal to the side of the prism (using a protractor or set square).

5.      From the point chosen in 4 above, draw three straight lines (representing rays of light) striking the side of the prism at angles of incidence of 30o, 45o and 60o.  Clearly label the angles of incidence.  These lines are going to be used to guide rays of light to strike the prism at those angles of incidence.

6.      Shine a single ray of light striking the prism at an angle of incidence of 30o and clearly trace onto the paper the path of the refracted ray emerging on the other side of the prism.  Repeat this for the other two angles.

7.      Measure and label the angles of refraction for each of the angles of incidence.  Tabulate these results in the table shown in the Results section.

8.      Complete all entries in the table and the other details required in the Results section.

RESULTS:

 Angle of Incidence (o) Angle of Refraction (o) Sin i Sin r Refractive Index 30 45 60 Average =

1.     Show how you calculated the refractive index of the prism from the 30o angle of incidence data.

2.     Determine an average value for the refractive index of the prism by calculating an average of all three refractive index measurements in the table.

3.      Suggest TWO methods for improving the reliability of this experiment.

CONCLUSION: Now write a simple conclusion for this practical.

### RESEARCH ASSIGNMENT

Choose ONE application of Physics related to waves from the following list:

¨      The Global Positioning System

¨      CD Technology

¨      The Internet (digital process)

¨      DVD Technology

For this application:

·        Identify (name) at least three different sources of information on this application.  Give full details of each source.

·        Gather, process and present information from secondary data sources to identify areas of current research on this application of physics.

·        Use the available information to discuss some of the underlying physical principles used in the application you have chosen.

This assignment paper is to be submitted for marking at the beginning of the Physics period on Tuesday 30/4/02.  Word limit: maximum of 500 words.

WARNING: Do not plagiarize.  Communicate the ideas in your own words.

DATE ISSUED:      