GRADE 9 SCIENCE
WAVES
Waves transmit many different types of energy eg light, heat, sound and radio energy.
Waves are caused by a disturbance such as a vibration or oscillation.
 Vibration or oscillation - regular, repetitive to and fro or up and down motion.
 Pulse - single disturbance that is propagated from a point in a medium.
TYPES OF WAVES
 Transverse wave - the medium moves at right angles to the wave direction.

Longitudinal wave - material it is passing through moves in the same
direction as the wave. Sound waves are longitudinal waves.
Direction of wave
VELOCITY, FREQUENCY AND WAVELENGTH
Frequency - the number of complete waves passing a given point in one second.
 Once a wave has been generated at a certain frequency, the frequency remains constant
irrespective of the medium through which it passes.
 Units - Hertz (Hz)
Wavelength - the distance between two corresponding points in phase.
 Once a wave has been generated, wavelength can change if the velocity changes.
 Units - metres (m)
Velocity - of a wave is the number of metres travelled in one second.
 Velocity can change when a wave passes from one medium into another of different density or,
in the case of water, a medium of different depth where there is friction between the wave and
the bottom.
 Units - metre per second (m.s-1).
Mathematical relationship: velocity = frequency x wavelength
v = f
v = velocity (m.s-1), f = frequency (Hz) and  = wavelength (m).
Calculation examples:
1.
Calculate the wavelength of a wave which has a velocity of 10 m.s-1 and a frequency of 0,5 Hz.
2.
Calculate the frequency of a wave that has a velocity of 20 m.s-1 and wavelength of 0,2 m.
3.
Calculate the velocity of a wave having a frequency of 10 Hz and wavelength of 0,2 m.
AMPLITUDE - Amplitude is the maximum displacement attained from the position of rest.

I
I

I
A
I
 = wavelength
A = amplitude
I
I
I
.
I
A
A
I

PERIOD AND FREQUENCY
Period - time taken for one complete wave to pass a given point.
Mathematical relationship: Period = 1/frequency
T=
1
f
T = period (s) and f = frequency (Hz).
Calculation example:
Calculate the period of a wave if its frequency is 20 Hz.
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WATER WAVES
REFLECTION
The direction of water waves is shown by means of a ray. A ray is a straight line drawn perpendicular
to a series of wave fronts.
Normal
Reflection by a plane surface.
i r
Reflecting surface
i = angle of incidence
r = angle of refection
Angle of incidence equals the angle of reflection.
REFRACTION OF WATER WAVES
Refraction - the bending of waves due to the change in velocity when they pass into water of
different depth.
Refraction of water waves by a rectangular piece of glass:
 Waves travel slower in shallow water than in deep water.
This is caused by greater friction between the wave and the bottom in shallow water.
 Since v = f, and f is always constant, a change in velocity results in a change in wavelength.
 When a wave passes from deep into shallow water, v and  both decrease.
 The part of the wave front that touches the glass first slows down ahead of the rest of the
wave front causing it to bend towards the normal.
 When a wave passes from shallow water into deep water, the v and  both increase.
 The part of the wave front that reaches deeper water first speeds up ahead of the rest of the
wave front causing it to bend away from the normal.
 This gives rise to refraction when the wave passes obliquely into water of different depths.
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DIFFRACTION OF WATER WAVES
Diffraction - bending of waves around an obstacle.
Diffraction by a single slit
 When water waves pass through a single slit, they form a circular wave pattern.
 The degree of diffraction is influenced by two factors;
(a)
Width of the slit - narrower the slit the greater the diffraction.
narrow slit:
(b)
wide slit:
Wavelength - longer the wavelength, the greater the diffraction.
Long wavelength
Short wavelength
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SUPERPOSITION OF WAVES
When two waves interfere with each other, a new wave is formed that is either bigger (constructive
interference) or smaller (destructive interference) than either of the original pulses.
Note:
We will only look at waves with the same wavelength.
A Destructive Interference: Two pulses on opposite sides of the rest position meet:
 The two pulses appear to cancel each other out but after that re-appear as they were before
meeting.
 On meeting, the pulses either cancel each other out completely if they were the same
amplitude or partially if they have different amplitudes.
B Constructive Interference: Two pulses on the same side of the rest position meet.
 There are always two pulses, even though when meeting it looks like there is just one.
 When the pulses meet, the resulting pulse is much bigger than either of the two.
A – Destructive Interference
B – Constructive Interference
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Interference pattern produced by two point source generators:
Lines of minimum disturbance (min)
are called nodal lines.
Lines of maximum disturbance (max)
are called anti-nodal lines
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Interference patterns from double slit diffraction
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