WAVES:-

Waves are one of the ways in which energy may

be transferred between stores. All waves transfer
energy but they do not transfer matter.
Oscillations
The repeated and regular fluctuations, above and
below the same position, eg the pressure of a
sound wave or the voltage of an alternating
current.
 sound waves cause air particles to vibrate
back and forth
 ripples cause water particles to vibrate up
and down
longitudinal waves
A wave that moves in the same direction as the
direction in which the particles are vibrating.
transverse waves
A wave that moves in a direction at right angles
to the way in which the particles are vibrating.
P-waves
Longitudinal seismic waves that are faster
moving and travel through liquids and solids.
They can therefore move through the crust,
mantel and inner and outer core of the Earth.

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S-waves
Transverse seismic waves that are slower moving
and travel through solids only. They can therefore
only move through the crust and mantle of the
Earth.
Medium
A material through which a wave can be
transmitted (propagate).
Parts of a wave
Waves are described using the following terms:
 rest position - the undisturbed position of
particles or fields when they are not vibrating
 displacement - the distance that a certain
point in the medium has moved from its rest
position
 peak (crest) - the highest point above the
rest position
 trough - the lowest point below the rest
position
 amplitude - the maximum displacement of a
point of a wave from its rest position
 wavelength - distance covered by a full
cycle of the wave, usually measured from
peak to peak, or trough to trough

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  time period - the time taken for a full cycle
of the wave, usually measured from peak to
peak, or trough to trough
 frequency - the number of waves passing a
point each second
 wavefront - the imaginary plane of waves
cutting through them in the same position.
This could be where all the peaks (crests)
are, all the troughs are or any place in
between.

Transverse waves
Waves cause a disturbance of the medium
through which they travel. This allows them to
carry energy. The quantity of energy carried
relates to the amplitude of the wave.
In transverse waves, the particles of the medium
vibrate at right angles to the direction that the
energy travels. This is where the name
transverse comes from - it means across. All of
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the waves in the electromagnetic spectrum are
transverse waves, as are water waves.
The following types of waves are transverse
waves:
 ocean waves
 light waves
 microwaves
 radio waves
 ultraviolet radiation
Transverse waves can also be made by shaking a
rope up and down or from side to side and look
like this:
Step 1:-
Producing transverse waves using a rope

A hand holds a length of rope taut

Step 2:-
A hand quickly moved down to produce a sine wave shape in the rope.

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Step 3:-
A hand qucikly moved up to continue the sine wave motion in the rope. The waves travel
from left to right, and the particles of the rope vibrate vertically.

In a transverse wave, the oscillation

(vibration) is at right angles to the
direction of energy transfer.
 The particles move up and down as the
wave moves from left to right.
 However, none of the particles are
transported along a transverse wave
Longitudinal waves
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 Sound waves, pressure waves and primary waves
(a type of seismic wave produced by earthquakes),
are longitudinal waves. Longitudinal waves
can also be made by pushing a spring forwards and
backwards. They look like this:
Producing longitudinal waves using a
spring
Step 1
A hand holding a stationary spring
stretched horizontally

Step 2
A hand quickly pushes forward to produce a
compressed area in the spring

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Step 3
A hand quickly pulls back to produce a
stretched area in the spring

Step 4
The compressed and stretched areas in the
spring travel along its length. The wave
travels from left to right, and the particles
of the spring vibrate horizontally.

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In a longitudinal wave, the vibration is
parallel to the direction of energy transfer.
In the diagram above, each individual coil
of the slinky moves backwards and
forwards either side of its rest position.
However, none of the coils are moved along
the length of the slinky. They just move
backwards and forwards parallel to the
direction in which the energy flows.
The energy is transferred along the
slinky, parallel to the oscillation.

Wave speed
The speed of a wave can be calculated
using the equation:
wave speed = frequency × wavelength
v=f λ

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This is when:
 wave speed (v) is measured in metres
per second (m/s)
 frequency (f) is measured in Hertz (Hz)
 wavelength (λ) is measured in metres
(m)

Example
What is the speed of a wave that has a
frequency of 50 Hz and a wavelength of 6
m?
v=fλ
v=50×6
v=300 m/s

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Question
What is the speed of a wave with a
frequency of 0.2 Hz and a wavelength of 25
m?
Show answer
Measuring waves in a ripple tank
A ripple tank can be used to investigate
the frequency, wavelength and the speed of
water waves.
A ripple tank is a transparent shallow tray
of water with a light shining down through
it onto a white card below.
The light allows you to see the motion of
the ripples created on the water's surface
more easily.
Ripples can be made by hand but to
generate regular ripples it is better to use a
motor.

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Reflection of waves
Waves - including water waves, sound and
light - can be reflected at the boundary
between two different materials.
When drawing wave diagrams, it is easier
to draw wave fronts rather
than crests and troughs.
Each wavefront is drawn at right angles to
the wave direction.

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For wave diagrams, the wave direction and
wave fronts are normally drawn.

The distance between two wave fronts is

the wavelength of the wave v=f λ
Waves, such as water waves, obey the law
of reflection that states:
angle of incidence (i) = angle of reflection
(r)
The angles of incidence and angle of
reflection are measured between the wave
direction and the normal – an imaginary line
drawn at 90° to the barrier.
The diagram below shows a water wave
reflected at a plane barrier.

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The diagram shows that when water waves
are reflected their direction changes, but
their wavelength and frequency remain
unchanged.
Key points
When waves are reflected:
 angle of incidence (i) = angle of
reflection (r)
 wavelength remains unchanged
 frequency remains unchanged
Refraction of waves
Refraction is the change in direction of a
wave as it travels from one medium to
another.

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For instance, when light travels from air to
glass or water waves travel from deep to
shallow water.

When water waves pass from deep to

shallow water they slow down
because friction with the ripple tank base
or seabed has greater impact in the shallow
water.
As the waves slow down the waves bend
towards the normal.
The frequency of the waves does not
change because the source of the waves
continues to vibrate at the same frequency.
Since v = fλ
If the speed (v) decreases and the
frequency (f) remains unchanged, then
the wavelength, λ, must also decrease.

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Key point
FAST is a useful way of remembering the
speed and direction changes of waves
during refraction:
 If waves get Faster, they bend Away
from the normal.
 If waves get Slower, they bend Towards
the normal.
Lett Meani
er ng
F Faster
A Away
S Slower
Towar
T ds
Diffraction
When waves meet a gap or an edge in a
barrier, they continue through the gap or
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past the edge of the barrier. However, what
happens on the far side of the gap or
barrier is not so straightforward.
The waves always 'spread' to some extent
into the area beyond the gap.
This is diffraction - the spreading out of
waves when they go through a gap, or past
the edge of a barrier.

Figure caption,
Waves passing through a narrow gap (gap
width less than the wavelength).
The extent of the spreading (diffraction)
depends on how the width of the gap
compares to the wavelength of the waves.
The wavelength is unchanged after
diffraction.

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A gap width similar to the wavelength of
the waves passing through causes a lot of
spreading, eg sound waves passing through
a doorway.
A gap width much larger than the
wavelength causes little spreading eg light
waves passing through a doorway.

Figure caption,
Waves passing through a wide gap (gap
width greater than the wavelength)
The extent of the diffraction also depends
on the wavelength of the waves.

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Figure caption:
Water waves passing a barrier
The greater the wavelength, the greater
the diffraction.
Diffraction affects radio and television
signals. Long wave radio signals are much
less affected by buildings, hills, tunnels etc.
than those of short wave or VHF radio, or
television.

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