Transcript Waves - Diffraction, Interference and Resonance

KS4 Physics
Diffraction, Interference
and Resonance
1 of 31
© Boardworks Ltd 2005
Contents
Diffraction, Interference
and Resonance
Diffraction
Interference
Natural frequency
Standing waves
Summary activities
2 of 31
© Boardworks Ltd 2005
The ripple tank
A ripple tank is a device used to study the behaviour of
waves, because all waves behave in a similar manner.
A ripple tank produces water
waves that can be reflected,
refracted and diffracted.
paddle vibrates
to produce
waves
3 of 31
© Boardworks Ltd 2005
Reflection and refraction
What do you think will happen if a barrier is placed in front of
the water waves?
paddle
If it is a plane barrier then
the waves are reflected.
What do you think will happen
if a block is submerged in the
ripple tank?
The change in depth of the
water causes a change in
speed of the waves – they
are refracted.
4 of 31
barrier
© Boardworks Ltd 2005
Barrier with small gap
What do you think will happen if a barrier with a gap in it
is placed in front of the water waves?
It depends upon the size of
the gap.
If the gap is smaller than the
wavelength of the waves what
do you think will happen?
The waves are reflected by the
barrier.
5 of 31
© Boardworks Ltd 2005
Barrier with wavelength-sized gap
What will happen if the width of the gap in the barrier is similar
in width to the wavelength of the waves?
Circular waves are
produced. This effect is
known as diffraction.
6 of 31
© Boardworks Ltd 2005
Barrier with large gap
What will happen if the width of the gap in the barrier is larger
than the wavelength of the waves?
The waves pass through the
gap unchanged apart from
slight diffraction of the waves
near their ends.
7 of 31
© Boardworks Ltd 2005
Effect of diffraction…
How can the teacher in the corridor hear the school band
even though he isn’t in the hall?
8 of 31
© Boardworks Ltd 2005
Effects of diffraction
Light and sound are both waves. Waves travel in straight lines.
When you are in a room, why can you hear people in the
corridor even though you can’t see them?
Light waves have a much shorter
wavelength than sound waves
and are not diffracted by the
doorway.
Sound waves have a wavelength
similar in magnitude to the width
of the doorway and so diffraction
occurs.
9 of 31
© Boardworks Ltd 2005
Contents
Diffraction, Interference
and Resonance
Diffraction
Interference
Natural frequency
Standing waves
Summary activities
10 of 31
© Boardworks Ltd 2005
Constructive interference
When two waves meet, they interfere with each other.
If they meet each other exactly in phase, the amplitudes
‘add up’ to produce large crests and troughs.
+
=
This is called constructive interference.
11 of 31
© Boardworks Ltd 2005
Destructive interference
If two waves meet each other exactly out of phase, the
amplitudes ‘subtract’ to produce no peaks or crests.
+
=
This is called destructive interference.
12 of 31
© Boardworks Ltd 2005
Young’s slits
To get two waves of light to interfere, the waves must be very
similar.
A single source of monochromatic
light is used, and split into two
waves by using a diffraction grating
like this:
In 1801, the English physicist Thomas Young first performed
this classic investigation, which showed the interference of
light waves.
13 of 31
© Boardworks Ltd 2005
Diffraction and interference
The light
source emits
rays of light,
which diffract
towards the
double slit.
S1
fringes
S2
S1 and S2 act as two
light sources.
The waves interfere:
constructively (bright fringes)
destructively (dark fringes)
14 of 31
© Boardworks Ltd 2005
Diffraction patterns
This is the front view of fringes produced by Young’s slits.
What would the fringes look like if white light was used as
the source instead?
15 of 31
© Boardworks Ltd 2005
Diffraction effects
The coloured
fringes on these
CDs are the result
of interference.
Light reflecting
from the aluminium
coating diffracts
and interferes.
Some colours are
diffracted more
than others.
16 of 31
© Boardworks Ltd 2005
Resonance
All objects have a natural frequency of vibration.
If an object if forced to vibrate at its
natural frequency it will vibrate at
its maximum amplitude.
This effect is called resonance.
The larger the mass of an object,
the lower its natural frequency.
What will happen to a glass made to
vibrate at its natural frequency?
Resonance causes it to shatter!
17 of 31
© Boardworks Ltd 2005
Waves, diffraction and interference
18 of 31
© Boardworks Ltd 2005
Contents
Diffraction, Interference
and Resonance
Diffraction
Interference
Natural frequency
Standing waves
Summary activities
19 of 31
© Boardworks Ltd 2005
Natural frequency
Fill a test tube rack with test tubes containing varying
amounts of water. Blow across the top of the test tubes.
What do you notice?
The shorter the air column, the higher the frequency.
The test tubes resonate at their natural frequency. How does
the length of air column affect its natural frequency?
The shorter the air column, the higher the natural frequency.
20 of 31
© Boardworks Ltd 2005
Mass and natural frequency
If these strings
are identical
apart from their
mass, which one
will have the
highest natural
frequency?
The smaller
the mass, the
higher the
natural
frequency.
The string with the smallest mass will have
the highest natural frequency.
21 of 31
© Boardworks Ltd 2005
Length and natural frequency
If these strings
are identical
apart from their
length, which one
will have the
highest natural
frequency?
The shorter
the string,
the higher
the natural
frequency.
The string with the shortest length will have
the highest natural frequency.
22 of 31
© Boardworks Ltd 2005
Tension and natural frequency
If these strings are
identical apart from
the tension they
are under, which
one will have the
highest natural
frequency?
10 N
15 N
20 N
The greater
the tension
on the
string, the
higher the
natural
frequency.
25 N
The string with the greatest tension will
have the highest natural frequency.
23 of 31
© Boardworks Ltd 2005
Changing natural frequency
24 of 31
© Boardworks Ltd 2005
Contents
Diffraction, Interference
and Resonance
Diffraction
Interference
Natural frequency
Standing waves
Summary activities
25 of 31
© Boardworks Ltd 2005
Standing waves
Some waves, such as light, move. Other waves, such as
those on a guitar string, do not move. These waves are
called standing waves.
Some parts of the standing waves actually move. These are
called antinodes.
Some parts of the standing waves do not move. These are
called nodes.
antinode
node
node
The lower the number of nodes,
the lower the frequency.
26 of 31
© Boardworks Ltd 2005
Standing waves and frequency
Which standing wave has the highest frequency?
1.
A
N
N
N
A
N
A
Wave 1 has the highest frequency,
because it has the most nodes.
A
2.
N
27 of 31
N
© Boardworks Ltd 2005
Contents
Diffraction, Interference
and Resonance
Diffraction
Interference
Natural frequency
Standing waves
Summary activities
28 of 31
© Boardworks Ltd 2005
Glossary
 antinode – The part of a standing wave that moves.
 constructive interference – When two in-phase waves






29 of 31
interfere to produce a larger wave.
destructive interference – When two out-of-phase
waves interfere to cancel each other out.
diffraction – The spreading out of waves as they pass
through a gap or round a barrier.
natural frequency – The frequency at which an object
resonates.
node – The part of a standing wave that does not move.
resonance – The vibration of an object at its maximum
amplitude when it is subject to its natural frequency.
standing waves – Waves that do not move.
© Boardworks Ltd 2005
Anagrams
30 of 31
© Boardworks Ltd 2005
Multiple-choice quiz
31 of 31
© Boardworks Ltd 2005