Download - Physics Form 4 Notes

Theme 3 -The Nature of Waves

Mr. N. Briffa B.Ed (Hons.)

1

Waves carry _______________ from one place to another. There are two kinds of waves:

a) Transverse waves b) Longitudinal waves.

Transverse waves are made up of _________________ and ____________________. Definition of a transverse wave: It is a wave in which the vibrations are _________________________ (900) to the direction of the wave.

Examples of transverse waves: ____________________, _________________, __________________.

Longitudinal waves are made up of ______________________ and ________________________. Definition of a longitudinal wave: It is a wave in which the vibrations are ________________________ (1800) to the direction of the wave.

Example of longitudinal wave: ______________________. http://www.youtube.com/watch_popup?v=Rbuhdo0AZDU



2

( ) S.I. unit: metre (m) The wavelength of a wave is the length of a single wave. It is denoted by the Greek letter (read as lambda). a) Transverse waves The wavelength ( ) for a transverse wave is the length of a single wave made up of a crest and a trough. However, it is also equal to the distance between two successive ______________ or two successive ___________________.

b) Longitudinal waves The wavelength ( ) for a longitudinal wave is the length of a single wave made up of a compression and a rarefaction. However, it is also equal to the distance between two successive ______________________ or two successive ___________________.

http://einstein.byu.edu/~masong/HTMstuff/WaveTrans.html

S.I. unit: metre (m)The displacement is the height of the wave, from its rest position. The maximum displacement is called __________________. So the amplitude is the height of a crest or the depth of a trough. The greater the amplitude, the greater the ________________ of the wave.

At which points is the displacement zero metres? _____________________________________ At which points do we measure the amplitude? _____________________________________

3 dimensional view



3

S.I. unit: Hertz (Hz)

Definition: It is the number of waves in _________ second. If 5 waves are generated in one second, then the frequency = ___________.

If 100 waves are generated in one second, then the frequency = __________.

The frequency of a wave can only change if the frequency of the source changes. If you dip your finger in water 3 times in 1 second, your frequency (the source) is 3Hz and the frequency of the waves is also 3Hz as 3 waves are produced every second. If the waves move in deep or shallow water the frequency will still be 3Hz as you would still be producing 3 waves in one second. Unless you change your frequency (source), the frequency of the waves will not change.

S.I. unit: seconds (s)

Definition: It is the time taken to complete one wave. If it takes 3 seconds to complete one wave, then the periodic time = ___________.

If it takes 0.2 seconds to complete one wave, then the periodic time = __________.

f = 1 and T = 1 T f Example:

If 5 waves are produced in one second, find: a) the frequency, b) the periodic time.

_______________________________________________________________________________________

_______________________________________________________________________________________



4 Frequency (f) =

no of waves in 1 second Periodic Time (T) = Time for one complete wave

Ex 1

Ex 2

Ex 3

Ex 4

Ex 5

Ex 6

Ex 7

Find the:

a) Periodic time _________________________________ b) Frequency __________________________________

Ex 8

Six waves hit a breakwater every minute. Find the: a) frequency ____________________________________________________________________ b) periodic time ____________________________________________________________________



5

Ex 9

The pendulum takes 2 seconds to swing from A to B. If the horizontal distance from A to B is 1m, find:

a) its amplitude of vibration _______________________________

b) its periodic time _______________________________________

c) its frequency __________________________________________

Ex 10

During an earthquake, the upper part of a skyscraper moves from side to side, a horizontal distance of 4m in 1.2 seconds. Find:



c) its frequency __________________________________________

Ex 11

The ruler is placed at the edge of a table and is set to vibrate as shown. If the end of the ruler moves a vertical distance of 3cm in 0.2 seconds, find:



c) its frequency __________________________________________

!S.I. unit: metres per second (m/s) This is the velocity with which the wave travels.

Velocity = Frequency x Wavelength v = f (m/s) (Hz) (m) Example 1:

Find the velocity of a wave having a frequency of 3Hz and a wavelength of 0.4m. ______________________________________________________________________________________________________________________________________________________________________________ Example 2:

Find the frequency of a wave which is moving at 2m/s and which has a wavelength of 30cm. ______________________________________________________________________________________________________________________________________________________________________________



6

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Gamma rays have the ____________________ frequency, the _____________________ energy and the

__________________________ wavelength.

Radio waves have the _____________________ frequency, the _______________________ energy and

the __________________________ wavelength.

Gamma rays, X-rays, Ultraviolet, Visible light, Infra-red, Microwaves and Radio waves are all

electromagnetic waves that make up the _____________________________________________. Except for

visible light, all the other waves are invisible to the human eye.

Common properties of Electromagnetic waves:

1. They are all __________________________ waves.

2. They do not need a medium to travel through, so they can travel in a ________________________.

3. They have a common ___________________ (3 x 108 m/s or 300,000,000 m/s).

4. Being waves they all carry __________________.

5. They obey the laws of reflection, refraction and diffraction.

6. They are uncharged. (not + or - )



7

Approximate (m)

Source Use Additional information

Gamma

Is very penetrating and can be very dangerous if used inappropriately. Can be detected with a Geiger-Muller tube.

X-rays

Produced when electrons hit a metal target. Can penetrate flesh but not bones and so produces a shadow, making fractures visible. Parts of the body need to be covered with lead. Can be detected by photographic plates.

Ultraviolet

Can be detected by using fluorescent chemicals making objects glow in the dark.

Visible light

The only radiation which we can see. Can be further divided into seven colours.

Infra-red

They can be detected by using a thermometer with a blackened bulb.

Microwaves

Microwaves make the water particles contained in food vibrate causing heating. They cause burns if absorbed by the body.

Radio waves

Information is encoded into a radio wave, transmitted to a receiver where it is decoded.



8

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Water waves are transverse waves. They can be studied in a __________________________. Architects and engineers use ripple tanks to study the best design for breakwaters before they start projects on a large scale. How do you produce straight waves in a ripple tank? By using a _____________________ that is forced to move up and down by an electric motor. How do you produce circular waves in a ripple tank? By using a _____________________ that is forced to move up and down by an electric motor. A _____________________ is an instrument that makes waves appear stationary. The ___________________ is equal to the distance between two successive wavefronts and is measured with a metre ruler.

$

The angle of incidence (i) is equal to the angle of reflection (r). Draw the normal and the reflected wavefronts in each diagram.

Underline the correct answer:

After the waves are reflected: a) the wavelength ( increases, remains the same, decreases) b) the frequency ( increases, remains the same, decreases) c) the velocity ( increases, remains the same, decreases)



9

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When water waves change the depth, they change direction because they change their _____________. Underline the correct answer: When the wavefronts pass from deep water to shallow water:

the wavelength (increases, remains the same, decreases) the frequency (increases, remains the same, decreases) the velocity (increases, remains the same, decreases) the wave changes direction and is refracted (towards, away from) the normal.

Underline the correct answer: When the wavefronts pass from shallow water to deep water:

the wavelength (increases, remains the same, decreases) the frequency (increases, remains the same, decreases) the velocity (increases, remains the same, decreases) the wave changes direction and is refracted (towards, away from) the normal.

http://www.youtube.com/watch_popup?v=r0088hYFuws



10

Complete the diagrams below.

http://www.youtube.com/watch_popup?v=stdi6XJX6gU

If the wavefronts enter the shallow water along

the normal, they are not refracted. However

the wavelength and the speed still

_______________ in shallow water and

_________________ in deep water. The

frequency does not change.



11

Question: A graph of the displacement against distance for a wave is shown in the diagram.

a) Using the graph, find the amplitude of the wave.

____________________________________________________________________________(1 mark)

b) Using the graph, find the wavelength of the wave.

____________________________________________________________________________ (1 mark)

c) If the frequency of the source is 100Hz, calculate the velocity of the wave.

____________________________________________________________________________(2 marks)

d) The wave enters a medium and slows down. What change, if any, is there in the frequency?

____________________________________________________________________________(2 marks)

e) What change if any is there in the wavelength?

____________________________________________________________________________(2 marks)

f) How would the same graph be different if the wave had more energy?

____________________________________________________________________________(2 marks)



12

When water waves pass through a gap they spread or __________________. The diffraction is greatest

when the gap width is similar to the wavelength of the wave. Spreading of waves is not desired in

harbours.

The narrow gap being similar to the wavelength of the waves causes more diffraction (spreading) of

waves and this would not be ideal for a harbour.

http://www.ngsir.netfirms.com/englishhtm/Diffraction.htm

http://www.youtube.com/watch_popup?v=4EDr2YY9lyA



13

Question:

Harbours have breakwaters to stop large waves.

a i) What is the wavelength of the water waves?

__________________________________________________________________________(2 marks)

ii) 5 waves hit the breakwater every 20 seconds. What is the frequency of the water waves?

________________________________________________________________________________

__________________________________________________________________________ (2 marks)

iii) Calculate the velocity of the waves.

__________________________________________________________________________ (2 marks)

b i) Complete the diagram above to show how the waves proceed after passing through the gap.

(2 marks)

ii) Explain why it would not be wise to make the gap too narrow.

________________________________________________________________________________

__________________________________________________________________________ (2 marks)



14

Question: This question is about an experimental design about water waves.

a) A tank 5 m long is filled with water. Using the apparatus above, describe how one could find the speed of the water wave.

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________(4 marks) b) A student is told that the deeper the water, the less the wave velocity. Describe how he should investigate this statement. Method:

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________(3 marks) Table of results:

(2 marks) Graph:

___________________________________________________________________________ (1 mark) Precautions:

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________ (3 marks) From the shape of the graph, how could the student tell if the statement is correct?

___________________________________________________________________________________

___________________________________________________________________________(2 marks)

stopwatch

measuring tape



15

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Sound energy travels in the form of longitudinal waves. A longitudinal wave is made up of _______________________ (C) and ____________________ (R). How does sound travel through air? By means of compressions and rarefactions of air particles. Sound waves from a loudspeaker produce compressions and rarefactions of invisible air molecules. A compression is a region in which the number of air molecules (particles) is high. A rarefaction is a region in which the number of air molecules is low.

The speaker has a cone which is made to vibrate in and out by an electric current. When the cone moves, out the air in front is compressed and when it moves in, the air is rarefied. Sound is not electromagnetic in nature as it requires a medium to travel. Sound does not travel in a vacuum. If an explosion occurs in space, nothing will be heard as there the sound cannot travel. Experiment: To show that sound requires a medium to travel.

As air is pumped out by a vacuum pump, the ringing of the bell gets lower and lower. When all the air is removed, the hammer can be seen vibrating but no sound is heard. This experiment shows that sound does not travel in a __________________.