Transcript What you will need to remember from year 10…

Demonstrate understanding of
aspects of wave behaviour
Physics A.S. 1.4
2013
Waves
• Waves are a means of transferring energy through a
medium without transferring matter
• Light and Sound travel as waves, in straight lines
• Speed of light 300,000Km/sec
• Speed of sound 300m/sec
• Electromagnetic waves form a large spectrum from
radio to gamma rays
Electromagnetic Spectrum
• The EMR energy increases as the wavelength gets shorter
• Low energy = communication, High energy = hazard to life
Waves
• There are many types of wave, but they are all
either:
• Longitudinal – particles vibrate/oscillate parallel
to the direction of the wave (eg Sound)
• Transverse – particles vibrate/oscillate at 90o to
the direction of the wave (eg water)
• The involvement of particles means these are
mechanical waves
• EMR waves e.g light are transverse waves that do
not need a medium (no particles are involved)
Understanding waves
Transverse waves
Longitudinal waves
compression and
rarefaction of
particles
Some definitions…
Crest
1) Amplitude –
measured from mid
position to crest or
trough
Trough
2) Wavelength () – this is the
distance between two
corresponding points on the
wave and is measured in metres:
3) Frequency – this is how many waves pass a point
every second and is measured in Hertz (Hz)
Terminology
•
•
•
•
•
Peak
Trough
Amplitude
Wavelength (λ)
Period (T) – time
taken for the
wave to travel 1
cycle OR time
for a wave to
pass a point,
measured in
seconds
1 cycle
TIME
Sound
We hear things when they vibrate.
If something vibrates with a high frequency (vibrates very
______) we say it has a _____ pitch.
If something vibrates with a low frequency (vibrates ______)
we say it has a ____ pitch.
Words – slowly, low, high, quickly
Higher frequency = Higher pitch
Frequency
This sound wave has a
high frequency:
_____
This sound wave has a
low _frequency:
___
What is the relationship between frequency and wavelength?
Lower frequency = higher wavelength
Amplitude
This sound wave has a
high amplitude
_____
(loud):
Amplitude = energy (volume, brightness etc)
This sound wave has a
low amplitude
_____
(quiet):
Sound video
Draw these waves:
1) Quiet sound, low frequency
(i.e. high wavelength):
2) Quiet sound, high frequency
(i.e. low wavelength):
3) Loud sound, low frequency:
4) Loud sound, high frequency:
The Wave Equation
The wave equation relates the speed of the wave to its
frequency and wavelength:
Wave speed (v) = frequency (f) x wavelength ()
in m/s
in Hz
in m
V
f

Questions
• Sound travels at 330m/s. If a sound note is played
at 484Hz find its wavelength.
• 0.68m (2-3 sig fig)
• The speed of light is 3.0x108 m/s and the
wavelength of red light is 650x10-9m. Find the
frequency of red light.
• 4.6x1014 Hz
• 4 complete water waves pass a point every 2 secs.
The distance between 6 crests is 12m. Find the
speed of the wave.
• 4.8m/s (draw the waves out)
Period and Frequency
• Period (T) –time for a wave to pass a point,
measured in seconds
• Frequency – how many waves pass a point every
second
• Period and frequency are related by the equation:
frequency (f) = 1 / period (T)
in Hz
in s
• F=1/T
• F is frequency (Hz)
• T is period (s)
1
f
T
Questions
• What is the period of a 330Hz wave?
• 0.003s
• What is the frequency of a wave with period
of 0.4s?
• 2.5Hz
• Complete the questions pg 113, 114 ESA study
guide
• Video – properties of waves
Reflection
• All waves can reflect.
• In a plane (flat) mirror
images are reflected
about the normal. (an
imaginary line 90o to
the mirror)
• Plane mirrors also
laterally invert images.
• Plane mirrors produce
virtual images (shown
by dashed line).
Reflection
Angle of incidence = Angle of reflection
Normal
Reflected ray
Incident ray
Angle of
incidence
Angle of
reflection
Mirror
Lateral Inversion
Images in plane mirrors appear the same size and as far
behind as in front but they are switched from “side to side”
L
R
Light from each hand reflects from the mirror into our
eyes. The eyes register the image in direct line of sight.
The persons left hand appears to be the right.
Why is the wave behind the mirror dashed?
Light is blocked by the mirror so the image is virtual.
How to draw reflection diagrams
• Draw the position of the reflected candle. (same size,
as far behind as in front)
• Draw 2 rays from the image to the eye (line of sight)
• Image must be virtual (dashed lines)
• Use the law of reflection to show how light travels
from the candle to the mirror. (normal line)
• Show the direction the rays move. (arrows in the
centre)
Refraction through a glass block:
How does the brightness of a refracted ray differ from a reflected
one? Why? Set up this experiment and complete the diagrams
Wave slows down and bends
towards the normal due to
entering a more dense medium
Wave speeds up and bends
away from the normal due to
entering a less dense medium
Wave slows down but is
not bent, due to entering
along the normal
Refraction
Rules
1. Describe the optical densities of the media
2. Frequency remains the same.
3. State the change of speed (faster or slower)
4. Therefore the wavelength changes (increase or decrease)
5. So the refracted ray bends (towards or away from the normal)
LESS
MORE

Refraction
• When going from less
optical density to more
it bends towards the
normal.
• When going from more
optical density to less it
bends away from the
normal.
• Refraction always
involves a change in
speed, direction and
wavelength.
Optical density changes: Air < water < glass < diamond
Refraction
Refraction is the bending of waves. It happens
when waves ____ __, wavelength and direction
due to entering a _________ (substance) of
different optical density. When a pen is placed
in water it looks like this:
In this case the light rays are slowed down by the water
and are _____, causing the pen to look odd. The two
mediums in this example are ______ and _______.
Draw a ray diagram to show
why the pen appears bent.
Words – change speed, water, air, bent, medium
http://www.bcscience.com/bc8/pgs/quiz_section4.2.htm
How to draw refraction diagrams
• Draw the image
• Draw 2 rays from the image to the viewer
(line of sight)
• Draw 2 rays from the object to the points of
refracted rays at the interface.
• Show the direction the rays move. (arrows in
the centre)
• Draw a normal to check refraction is correct
• wiki
Question
• Explain how the
prism creates a
spectrum.
• White light travelling through
air enters a more optically dense
medium (plastic) and is
refracted towards the normal
because it changes its speed,
direction and wavelength, as it
exits the prism it enters a less
optically dense medium (air) and
again is refracted this time away
from the normal. The shorter
wavelength blue light travels
faster and is bent more than the
longer wavelengths so a
spectrum is formed.
Finding the Critical Angle…
1) Ray gets refracted
3) Ray still gets refracted (just!)
THE CRITICAL
ANGLE
2) Ray still gets refracted
4) Ray gets
internally reflected
Total Internal Reflection
• If light goes from a more optically dense medium to a less optically
dense one it is refracted away from the normal. Θi < Θr
• When the angle of refraction reaches 90o the angle of incidence is
known as the critical angle.
• Any further increase in the angle of incidence will result in the
light being reflected rather than refracted.
Rules
• More optically
dense to less
• Θi = Θc when
refraction is at
90o
• Θi > Θc
therefore
reflection
Mirages
• The mirage is a
reflection of the
sky. Explain using
the ideas of total
internal reflection
how the mirage
occurs. In your
answer consider
how the optical
density of the air
changes as it gets
hotter.
• Air near the road surface is heated and
less optically dense. Light from the sky
therefore travels from a more to a less
optically dense medium and bends away
from the normal. The Θi = Θc when
refraction is at 90o. In this case the Θi >
Θc therefore reflection of the sky
occurs and the brain misinterprets it as
water.
Uses of Total Internal Reflection
Optical fibres:
An optical fibre is a long, thin, _______ rod made of
glass or plastic and sheathed in a low optical density
material.
When light hits the boundary the angle of incidence is
greater than the critical angle and so is _______
reflected from one end to the other, making it
possible to send ____ chunks of information
Optical fibres can be used for _________ by sending
electrical signals through the cable. The main advantage
of this is a reduced ______ loss.
Words – communications, internally, large, transparent, signal
Other uses of total internal reflection
1) Endoscopes (a medical device used to see inside the body):
2) Binoculars and periscopes (using “reflecting prisms”)
Complete All
questions in
activity 8B pg 130
ESA study guide
Rainbows
• In order to see a
rainbow the
observer must be
looking towards the
rain with the sun
behind them.
Refraction and Dispersion
• White light can be
refracted and dispersed
into the visible spectrum.
• When light enters a more
optically dense medium it
refract, slows down,
decreases wavelength and
bends towards the normal.
• Red light slows less than
violet and decreases its
wavelength by less
therefore it bends less
towards the normal.
• When travelling into a less
optically dense media the
opposite occurs.
In a prism light bends twice in
the same direction (down and
Down)
Wave diagrams
1) Reflection
2) Refraction
3) Refraction
4) Diffraction
Diffraction
Diffraction is the ability of waves to spread from a gap or get
around a barrier. (don’t use the term bending)
More diffraction if the size of the gap is similar to the wavelength
More diffraction if wavelength is increased (or frequency decreased)
Low frequency sounds diffract more
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/imgsou/difr.gif
Diffraction over or around the hill
• If a man and woman are at the source whose
voice would the child hear?
• Man has low pitch voice
• Low frequency, high wavelength because V=f
• So mans voice diffracts more and is heard
CHILD
http://www.pa.op.dlr.de/acoustics/essay1/beugung_en.html
Radio wave diffraction
• FM radio is high
frequency
• v = fx
• Therefore short
wavelength which can’t
diffract much so the
house doesn’t get
reception
• AM radio is low
frequency
• Therefore long
wavelength which
diffracts more so the
house gets reception
Sound refraction
• Sound travels faster in warm air.
• Lower part of wave front gets ahead of
upper part, so front turns upward.
http://sol.sci.uop.edu/~jfalward/physics17/chapter10/chapter10.html
ripple tank demo
Sound reflections (echos)
• When sound waves hit a
surface they are reflected.
• They are travelling in the
same medium so they will
have the same frequency,
speed and wavelength BUT
the direction will change.
• The amplitude of reflected
wave is less as some of the
energy is “lost”.
• V = d/t
V= speed (m/s)
D = distance (m)
T = time (s)
Sonar can also be used to tell the
different layers beneath the sea floor.
http://www.coml.org/edu/tech/count/acoustic-tech-obs/bottom-penetrating-sounders/2-dositsreflec.gif
Application - Echolocation in mammals
• Some animals use ultrasound to find out about their
surroundings, navigate and find food and mates.
• High pitch, low energy clicks reflect off close objects.
• The animal detects and determines how far away an object
is based on the time difference.
• Advantages
–
–
–
–
High frequency, can’t be heard by predators
Short wavelength, more likely to reflect off rather than diffract
Doesn’t require much energy to produce the clicks
Energy dissipates quickly, less interference of signal
• Disadvantages
– Energy dissipates quickly, no good for far away objects
How does ultrasound work?
Ultrasound is the region of sound above 20,000Hz – it can’t
be heard by humans. It can be used in pre-natal scanning:
How does it work?
Ultrasonic waves are partly _________ at the boundary as they pass from
one _______ to another. The time taken for these reflections can be
used to measure the _______ of the reflecting surface and this
information is used to build up a __________ of the object.
Words – depth, reflected, picture, medium
Other uses of ultrasound
1) Echo sounding
The ultrasound is reflected from the
sea floor.
2) Breaking down kidney stones
Ultrasonic waves break kidney
stones into much smaller pieces
3) Cleaning (including teeth)
Ultrasound causes dirt to vibrate
dirt off without damaging the object
The Structure of the Earth
A thin crust 10-100km thick
A mantle – has the
properties of a solid
but it can also flow
A core – made of
molten nickel and iron.
Outer part is liquid
and inner part is solid
How do we know this? These facts have all been
discovered by examining seismic waves (earthquakes)
Seismic waves
Earthquakes travel as waves through the Earth – we call them
SEISMIC WAVES. There are two types:
P waves:
1) They are longitudinal so they cause the ground
to move up and down
2) They can pass through solids and liquids
3) They go faster through more dense material
S waves:
1) They are transverse so they cause the ground
to move from right to left
2) They ONLY pass through solids
3) They are slower than P waves
4) They go faster through more dense material
Seismic waves
S waves will only travel
through a solid
P waves travel through
the Earth and are
refracted when they
pass through a medium
The paths of these
waves are all curved
because density is
gradually changing
These observations tell us 3 things about the Earth: 1) It has a thin
crust, 2) it has a semi-fluid mantle where density increases with depth,
3) a core with a liquid outer part and a solid inner part.
The electromagnetic spectrum
Each type of radiation shown in the electromagnetic spectrum has a
different wavelength and a different frequency:
High frequency,
_____ wavelength
Gamma
rays
X-rays
Low frequency, _____
(high) wavelength
Ultra violet
Visible
light
Infra red
Microwaves
Radio/TV
γ
Each of these types travels at the same speed through a _______
(300,000,000m/s), and different wavelengths are absorbed by different
surfaces (e.g. infra red is absorbed very well by ___________ surfaces).
This absorption may heat the material up (like infra red and _______) or
cause an alternating current (like in a __ _______).
Words – black, microwaves, long, short, TV aerial, vacuum
How Microwaves and Infra-red work
Microwaves are absorbed by
water molecules up to a depth of
a few centimetres. The heat
then reaches the centre of the
food by conduction.
Infra-red waves are absorbed by
the surface of the material and
the energy is then passed to the
centre of the food by
conduction.
The higher the frequency of the
wave, the greater its energy
X-rays and gamma (γ) rays
X-rays are absorbed by ____ parts of the body, like ____.
Unfortunately, over-exposure to x-rays will damage cells.
Gamma rays can be used to treat _______. A
gamma ray source is placed outside the body
and rotated around the outside of the tumour.
Doing this can ___ the cancerous cells without
the need for ______ but it may damage other
cells and cause sickness.
Tracers can also be used – these are small amounts of
___________ material that can be put into a body to see
how well an organ or ______ is working.
Words – radioactive, gland, cancer, hard, bones, kill, surgery