WAVES [3.2 Light, Sound] except EM Flashcards
transverse waves e.g.s
define
infra-red waves, light waves, ultraviolet waves
[Ripples on the surface of water
Vibrations on a guitar string
S-waves (a type of seismic wave)
Electromagnetic waves (such as radio, light, X-rays etc)]
Waves where the points along its length vibrate at 90 degrees to the direction of energy transfer
For transverse wave:
ONLY EM WAVES TRAVEL IN VACUUM
constant density
pressure constant
speed of wave is depending on material it is travelling in
energy transfer is perpendicular to wave motion
They transfer energy, but not the particles of the medium
They can move in solids and on the surfaces of liquids but NOT inside liquids or gases
Some transverse waves (electromagnetic waves) can move in solids, liquids and gases and in a vacuum
LONGITUDINAL waves!
Waves where the points along its length vibrate parallel to the direction of energy transfer
For a longitudinal wave:
CANNOT travel in a VACUUM
changes in density
changes in pressure
speed of wave is depending on material it is travelling in
The energy transfer is in the same direction as the wave motion
They transfer energy, but not the particles of the medium
They can move in solids, liquids and gases
They can not move in a vacuum (since there are no particles)
📌 The key features of a longitudinal wave are where the points are:
Close together, called compressions
Spaced apart, called rarefactions
Examples of longitudinal waves are:
Sound waves
P-waves (a type of seismic wave)
Pressure waves caused by repeated movements in a liquid or gas
Longitudinal waves are usually drawn as several lines to show that the wave is moving parallel to the direction of energy transfer
Drawing the lines closer together represents the compressions
Drawing the lines further apart represents the rarefactions
Wave behaviour - Reflection, Refraction & Diffraction
Reflection occurs when:
A wave hits a boundary between two media and does not pass through, but instead stays in the original medium
law of reflection states
The angle of incidence = The angle of reflection
FIRST SIDE OF RAY COMING IN
angle of incidence
Refraction
occurs when:
When waves enter a different medium, their speed can change
A wave passes a boundary between two different transparent media and undergoes a change in speed
When a wave refracts, as well as a change in speed, the wave also undergoes:
A change in wavelength (but frequency stays the same)
A change in direction
;; Waves can change direction when moving between materials with different densities
If the waves slow down
the waves will bunch together, causing the wavelength to decrease
The waves will also start to turn slightly towards the normal
If the waves speed up
waves will spread out, causing the wavelength to increase
The waves will also turn slightly away from the normal
Diffraction
DIFFRACTION, REFRACTION, REFLECTION - all “wave effects”
When waves pass through a narrow gap, the waves spread out
When drawing waves being reflected take care to:
Make sure that the angle of incidence is equal to the angle of reflection
Keep the wavelength of the waves the same
Similarly, when waves are diffracted the wavelength remains constant.
Refraction is the only wave effect in which the wavelength changes.
Remember:
Refraction is the name given to the change in the speed of a wave when it passes from one medium to another. The change in direction is a consequence of this.
Factors Affecting Diffraction
-> extent of diffraction depends on the width of the gap compared with the wavelength of the waves
Diffraction is the most prominent when the ______________
MOST prominent when width of the slit is approximately equal to the wavelength
As the gap gets bigger
the effect gradually gets less pronounced until, in the case that the gap is very much larger than the wavelength, the waves no longer spread out at all
Diffraction can also occur when waves pass an edge
When a wave goes past the edge of a barrier, the waves can curve around the edge
Investigating Waves with a Ripple Tank
Ripple tanks are commonly used in experiments to demonstrate the following properties of water waves:
Reflection at a plane surface
Refraction due to a change in speed caused by a change in depth
Diffraction due to a gap
Diffraction due to an edge
Investigating Reflection
Reflection can be shown by the waves hitting a plane (straight) surface, such as a wall or mirror
Investigating Refraction
SHALLOW water = DECREASE in wavelength, SLOWER
When water waves travel from deep areas to shallow areas they slow down
Refraction can be shown by placing a glass block in the tank
The glass block should sit below the surface of the water and cover only some of the tank floor
The depth of water becomes shallower here the glass block is placed
Since speed depends on depth, the ripples slow down when travelling over the block
This is a good model of refraction showing how waves slow down when passing from deep water into shallow water
Investigating Diffraction
Diffraction can be shown in a ripple tank by placing small barriers and obstacles in the tank
As the water waves encounter two obstacles with a gap between them, the waves can be seen to spread out as follows:
amount of diffraction depends
depends on the size of the gap compared to the wavelength of the water wave
DIFFRACTION;; how the wavelengths differ with frequency in a ripple tank
The higher the frequency of the motor, the shorter the wavelength
The lower the frequency of the motor, the longer the wavelength
LIGHT!!
n = sin i/sin r!!!!
N = SIN I/SIN R!!!!!!!!!!!
E.G. : A ray of light from a laser passes from air into a clear, semi-circular, plastic block. Fig. 7.1
shows the ray entering the block.
The ray continues in the same direction and meets the middle of the opposite surface at an
angle of 40 ° to the normal. The refractive index of the plastic is 1.5.
(a) The ray continues into the air.
Calculate the angle between the normal and the path taken by the light after it leaves
the block.
FINDING R given air is lower density so lower n - GIVEN DIFFERENT N & SIN I
so find: r, the n we need, then sin inverse the n…?
n = sin i/sin r
0.9641
75/74.6° to 2 or more sig. figs.
[3 MARKS]
anw working:
-> USING N = SIN I/SIN R;; sin^-1 (sin 40/1.5)
r = 25
[THIS USES DIFFERENT n VALUE SO NOT IT!]
SNELLS LAW: r = sin inverse(sin 40 x 1.5)
= 74.6 degrees
Explain why the ray does not change direction as it enters the plastic block. [2]
wave hits/enters the plastic at the same time OR incident ray perpendicular along normal
wave all slows down at the same time OR refracted ray perpendicular normal
RAY DIAGRAMS
Angles are measured between the wave direction (ray) and a line at 90 degrees to the boundary
The angle of the wave approaching the boundary is called the angle of incidence (i)
The angle of the wave leaving the boundary is called the angle of reflection (r)
The line at right angles (90°) to the boundary is known as the normal
When drawing a ray diagram an arrow is used to show the direction the wave is travelling
An incident ray has an arrow pointing towards the boundary
A reflected ray has an arrow pointing away from the boundary
Reflection in a Plane Mirror
When an object is placed in front of a mirror, an image of that object can be seen in the mirror
describe image reflection in mirror
image will be:
The same size as the object
The same distance behind the mirror as the object is in front of it
DIRECTLY IN LINE WITH OBJECT
Virtual
Light from the object hits the mirror, reflecting from it (i=r)
To an observer, the reflected ray appears to have come from the right-hand side of the mirror
The reflected ray can be traced back in this directions, forming a virtual ray
can be repeated for another ray travelling in a slightly different direction
An image of the object will appear where these two virtual rays cross
virtual image
A virtual image is formed by the divergence of rays from the image,
and cannot be projected onto a piece of paper (because the rays don’t actually go through the image)
radio waves
radio and television communications
microwaves
satellite television & telephone
infra-red
electrical appliances, remote controllers for televisions & intruder alarms
x-rays
medicine & security
focal length of the lens
first intersection with principal axis
to centre of the lens
n = sin i/sin r
then v = c/n
Describe how glass fibres are used in communications technology. [3]
infra-red encoded.
[signal OR data OR internet]
(optical fibre transmits) light
total internal reflection (prevents escape)
State and explain whether the image in (a) is real or virtual.
image is virtual/not real AND
cannot be seen on screen OR no rays come from (position of) image
The refractive index of water is 1.33.
Calculate the critical angle.
critical angle =
sin c = 1/n (refractive index)
Describe, with a diagram, a medical use of optical fibres [3]
appropriate use, accept diagram
accept ‘endoscope’
clear diagram of the above use or t.i.r. diagram for optical fibre
one from:
light goes down fibre/into body
illuminates internal organ
light/image returns from body/organ
Waves
repeated vibrations that transfer energy
transfer energy and information
described as oscillations or vibrations about a fixed point
[e.g., ripples cause particles of water to oscillate up and down. Sound waves cause particles of air to vibrate back and forth]
In ALL cases,
waves transfer energy without transferring _____
WITHOUT transferring matter
which means, for water waves, this means it is the wave and not the water (the matter) itself that travels
For sound waves, this means it is the wave and not the air molecules (the matter) itself that travels
Proof
Objects floating on water provide evidence that waves only transfer energy and not matter
The diagram below shows a toy duck bobbing up and down on top of the surface of some water, as waves pass it underneath.
Explain how the toy duck demonstrates that waves do not transfer matter.
Step 1: Identify the type of wave
The type of wave on the surface of a body of water is a transverse wave
This is because the duck is moving perpendicular to the direction of the wave
Step 2: Describe the motion of the toy duck
The plastic duck moves up and down but does not travel with the wave
Step 3: Explain how this motion demonstrates that waves do not transfer matter
Both transverse and longitudinal waves transfer energy, but not the particles of the medium
This means when a wave travels between two points, no matter actually travels with it, the points on the wave just vibrate back and forth about fixed positions
Objects floating on the water simply bob up and down when waves pass under them, demonstrating that there is no movement of matter in the direction of the wave, only energy
for a transverse wave, the
direction of vibration is at right angles to the
direction of propagation (movement)
and understand that
electromagnetic radiation, water waves and
seismic S-waves (secondary) can be modelled as
transverse
key distinction between the particles (or oscillations) of a wave, and the wave itself.
The motion of the wave causes the particles to move. The particles themselves are not the wave.
WAVE MOTION;; Wave vibrations can be shown on…
ropes (transverse) and springs (longitudinal)
Demonstrating Wave Motion
Properties of waves, such as frequency, wavelength and wave speed, can be observed using water waves in a ripple tank
Wave motion of water waves may be demonstrated using a ripple tank
The wavelength of the waves can be determined by:
Using a ruler to measure the length of the screen
Dividing this distance by the number of wavefronts
The frequency can be determined by:
Timing how long it takes for a given number of waves to pass a particular point
Dividing the number of wavefronts by the time taken
The wave speed can then be determined by:
Using the equation wave speed = frequency × wavelength
Features of a Wave
- crest/peak
The highest point on a wave above the equilibrium, or rest, position
- trough
The lowest point on a wave below the equilibrium, or rest, position
- amplitude
The distance from the undisturbed position to the peak or trough of a wave
symbol A and is measured in metres (m)
maximum or minimum displacement from the undisturbed position
- wavelength
The distance from one point on the wave to the same point on the next wave
In a transverse wave:
In a longitudinal wave:
TRANSVERSE - The wavelength can be measured from one peak to the next peak
LONGITUDINAL - The wavelength can be measured from the centre of one compression to the centre of the next
symbol λ (lambda) and is measured in metres (m)
frequency
The number of waves passing a point in a second
symbol f and is measured in Hertz (Hz)
Wave speed is the:
- speed at which energy is transferred through a medium
- The distance travelled by a wave each second
symbol, ν, and is measured in metres per second (m/s)
v=fwavelength
Wavefronts are a useful way of picturing waves from above:
each wavefront is used to represent a single wave
The arrow shows the direction the wave is moving and is sometimes called a ray
The space between each wavefront represents the wavelength
When the wavefronts are close together, this represents a wave with a short wavelength
When the wavefronts are far apart, this represents a wave with a long wavelength
undisturbed position
centre of the wave
amplitude is from centre of wave to top [in centre of wave]
equation relating time period and frequency
T = 1/f
1 kHz =
1000 Hz
kHz -> Hz
x 1000
approximate range of frequencies
audible to humans as 20 Hz to 20 000 Hz
a medium is needed to transmit sound
waves
speed of sound in air is
approximately 330–350 m/s
Sound travels fastest in solids
Sound travels slowest in gases
350 M/S IN GAS
1500 M/S IN LIQUID
5000 M/S IN SOLID
SPEED OF SOUND = distance sound travels/time taken
also = 2 x distance to wall/time
also = distance btwn microphones/time btwn peaks
In the case of measuring the speed of sound:
Method 3 is the most accurate because the timing is done automatically
Method 1 is the least accurate because the time interval is very short
The frequency of a sound wave is related to its pitch
Sounds with a high pitch have a high frequency (or short wavelength)
Sounds with a low pitch have a low frequency (or long wavelength)
The amplitude of a sound wave is related to its volume
Sounds with a large amplitude have a high volume
Sounds with a small amplitude have a low volume
Ultrasound is the name given to sound waves with a frequency greater than 20 000 Hz
When ultrasound reaches a boundary between two media, some of the waves are partially reflected
The remainder of the waves continue through the material and are transmitted
Ultrasound transducers are able to:
Emit ultrasound
Receive ultrasound
The time taken for the reflections to reach a detector can be used to determine how far away a boundary is
This is because ultrasound travels at different speeds through different media
This is by using the speed, distance, time equation
allows ultrasound waves to be used for both medical and industrial imaging
Ultrasound in Medicine
In medicine, ultrasound can be used:
To construct images of a foetus in the womb
To generate 2D images of organs and other internal structures (as long as they are not surrounded by bone)
As a medical treatment such as removing kidney stones
An ultrasound detector is made up of a transducer that produces and detects a beam of ultrasound waves into the body
The ultrasound waves are reflected back to the transducer by boundaries between tissues in the path of the beam
For example, the boundary between fluid and soft tissue or tissue and bone
Ultrasound in Industry
In industry, ultrasound can be used to:
Check for cracks inside metal objects
Generate images beneath surfaces
A crack in a metal block will cause some waves to reflect earlier than the rest, so will show up as pulses on an oscilloscope trace
Each pulse represents each time the wave crosses a boundary
The speed of the waves is constant, so measuring the time between emission and detection can allow the distance from the source to be calculated
In the diagram above, a very high-frequency sound wave is used to check for internal cracks in a large steel bolt. The oscilloscope trace shows that the bolt does have an internal crack. Each division on the oscilloscope represents a time of 0.000002 s. The speed of sound through steel is 6000 m/s.
Calculate the distance, in cm, from the head of the bolt to the internal crack.
Step 1: List the known quantities
Speed of ultrasound, v = 6000 m/s
Time taken, t = 5 × 0.000002 = 0.00001 s
Step 2: Write down the equation relating speed, distance and time
distance, d = v × t
Step 3: Calculate the distance
d = 6000 × 0.00001 = 0.06 m
Step 4: Convert the distance to cm
d = 6 cm
Explain, in terms of wave theory, what occurs as the wavefront strikes the boundary.
new wave generated
same speed
angle of incidence = angle of reflection
Sound travels in water as a series of compressions and rarefactions.
Describe what is meant by a compression and by a rarefaction
compression: closer together, particles/wavefronts closer together
rarefaction: further apart, particles/wavefronts further apart
Compressions and rarefactions occur along the path of sound waves.
State, in terms of the behaviour of molecules, what is meant by
compression
rarefaction
(region) where air molecules are
closer, more dense
(region) where air molecules are farther apart, pressure reduced
The boat passes over a region of the sea bed of the same depth, where the reflection of
sound waves is weaker.
State whether there is an increase, a decrease or no change in the amplitude and pitch
of the reflected wave.
amplitude: decrease
pitch: no change
State the effect on what is heard by a listener when there is
(i) an increase in the amplitude of a sound, [1]
(ii) a decrease in the wavelength of a sound.
i) louder sound
ii) sound has a higher pitch
State an assumption you made when calculating this distance.
light travels instantaneously
negligible wind / “no wind”
Describe how the movement of the loudspeaker cone produces these regions of different
pressure.
higher pressure
lower pressure
cone pushes air particles closer together as cone moves forward
cone moves backwards, causing empty spaces
During a thunderstorm, thunder and lightning are produced at the same time.
(a) A person is some distance away from the storm.
Explain why the person sees the lightning before hearing the thunder.
light travelling (much) faster than sound
+++
distance/3.6
3.6 = value, at start of storm
light vs sound waves -
longitudinal
transverse
electromagnetic
mechanical???
LIGHT: transverse, electromagnetic
SOUND: longitudinal, mechanical
State an approximate value for the speed of sound in air.
approximately 330 m/s
DONOTUSEOTHERVALUE
The man with his ear to the railway line actually hears two sounds from the hammer,
separated by a short interval.
Explain why he hears two sounds
sound through air and sound through steel
speeds in air and steel are different
Describe one use of optical fibres in medicine. You may draw a diagram. [3]
Light travels down (optic) fibres into or out of body
Endoscope inserted into body
To view internal organ body part
speed of light in air
Speed of light in air is 2.9 × 108 m/s.
Explain, in terms of diffraction, why a car radio may pick up low frequency radio signals
but not pick up high frequency radio signals when the car is travelling behind a hill. [2]
high frequency signals have shorter wavelength [1]
low frequency / short wavelength signals diffract less [1]
State what is meant by monochromatic light.
light of a single wavelength / frequency
Explain why the ray does not leave the fibre at Y [2]
optical fibres
angle of incidence at Y greater than critical angle
total internal reflection occurs
Fig. 6.1 shows white light incident at P on a glass prism. Only the refracted red ray PQ is
shown in the prism.
(a) On Fig. 6.1, draw rays to complete the path of the red ray and the whole path of the
violet ray up to the point where they hit the screen. Label the violet ray. [3]
red ray refracted away from normal
violet ray refracted more than red ray in prism
violet ray further refracted from red ray to screen
Describe the motion of a group of air particles situated on the path of the wave shown in
Fig. 7.1.
oscillation/vibration/backwards and forwards
along PY (consider pressure waves as alternative)
State why a person standing at point Y hears an echo. [1]
Sound reflects off wall
Sound waves are longitudinal waves.
State what is meant by the term longitudinal. [1]
vibration along direction of wave
[the direction of vibration is parallel to the direction of
propagation]
transverse: oscillations perpendicular to direction of travel of the wave
In terms of the wavelength, what is the distance along the wave between a
compression and the next rarefaction?
half a wavelength
Explain what is meant by
(i) the frequency of vibration of the strip,
(number of complete) vibrations (of the strip) per second/unit time
the amplitude of vibration of the end of the strip.
maximum displacement of end of strip from mid-position
XZ ÷ 2
Explain what is meant by
(i) total internal reflection
Reflection in a more dense material where there is no refracted ray
critical angle
The greatest angle of incidence (in the material) at which refraction occurs
